As I understand it, and I’m sure I’m wrong, some networks would require so many episodes or seasons before a show could be released to syndication. This kind of prevented the production company of double dipping or diluting the audience. If a show went into syndication after one season there would be people watching the new episodes on one channel and another watching reruns on another channel so dividing the audience and the important advertising dollars.

As an example, Star Trek the next generation was produced as a syndicated show. A production company paid for the whole show and sold the episodes to a network. I want to say it aired on CBS but it might have been Fox, but the reruns started on what would be considered the UHF channels, for a lack of a better term, after the first season. The production company did this by selling the episodes below cost knowing the would make it up on syndication. Towards the end of its run it cost over a million an episode which was outrageous at the time. It was selling the episodes for about $800,000 to the network and the $400,000 per episode in syndication making more money overall.

Star Trek:Enterprise was produced by Paramount and aired exclusively on UPN, a network channel owned by Paramount, but was co-produced by another production company. Their contract required 4 seasons before syndication and Star Trek shows made a ton of money in syndication. The business model for this show pretty much counted on hitting syndication. The UPN canceled the show at the end of the 2nd season or midway through the 3rd season. The production company lowered the cost per episode, essentially paying UPN to air the show so they could stay on for a 4th season to hit syndication

so typically, tv shows are exclusive to one network or provider. once it goes into syndication, it basically means that other networks buy the rights to air reruns of that show. so it's a way for the original producer of the show to continue to make money. in broadcast tv, they usually have a set schedule of when tv shows air, so they need lots of episodes in order to create a daily/weekly schedule for this show. nowadays with streaming, it doesn't matter as much. any show can be leased to any other streaming provider.

Originally (1940s-late 1950s or early 1960s) TV shows were shown just for that time slot. They would perform many of them live and not record them. They’d do 2 performances—one for the east coast and then a few hours later another live performance for the west coast. They were lost to time, not having been recorded. Eventually they did start recording them (I Love Lucy was one of the first) but there wasn’t a plan to do anything with them later. Recording just made it easier to show on the west coast without having to re-perform it. And then they could do re-runs later in the year. But because they used to do all the shows live, they did a lot of shows—like 36 shows a year, as compared to modern 22 shows or less/year.

Then, in the 1960s, they found a lucrative new way to make money—they would show old TV shows on another channel. To do this they would bundle up the existing shows and sell the rights to air them. There were only 3 channels but then they started expanding to new channels on UHF—these were channels larger than number 13. All the regular channels of NBC, CBS, and DuMont, then later ABC, were on channels 2 - 13. The UHF channels tended to have independent stations, not networks. They’d show local, cheap programming. Bad movies. Local news. Reruns purchased to be syndicated to these smaller channels.

Once the financial rewards of syndication were known, it was soon discovered that 5 seasons provided a good number of episodes for a syndication package, so often networks would continue to pay for shows to get to that 5th season so they’d have a lucrative package. Other shows that are shorter do syndicate, but might not get as good of a financial offer for it.

Happy Days is owned by x. They sell the rights to the show to smaller, local stations.

Local station in 1985 has 3 competitors. Local station pays too dollar to lease Happy Days because they want more viewers and can charge more to advertisers.

Advertisers are willing to pay more to be watched by more people. But especially people in the 16-35yo male demographic. And that’s who was watching Happy Days in 1985.

Plus it was better than what’s on the other 3 channels (which were probably Donahue, local news, and After School Specials).

Everyone wins.

It does still exist today. I challenge you to watch Extended Cable for 3 daylight hours and not see one episode of Big Bang Theory.

TLDR: Syndication is another word for re-run, and that's how major TV networks made their money on shows.

TV networks make money on ads, selling time on the air to run commercials. Without that revenue the network couldn't afford to pay anyone let alone the cost of making shows. But you can't rely 100% on original and new content, you need shows to fill time on the air.

Syndication meant re-airing the same episode of a show over and over again usually in less desirable time slots like during the day or after school.

In this way you could sell ads associated with those slots and make more money on a show that you had already paid for a long time ago. So even though you could sell a single Ad on a new episode during prime time for a lot of money, in the long run you would make far more money selling ads on re-runs of the show.

Syndicated TV shows could also be sold to other networks including foreign stations, getting even more money for the shows you created.

But to have a show available for syndication meant having at least 3 seasons worth so that you could air 5 episodes a week without repeating them for months otherwise it would get boring quick.

Even shows that did poorly would often get a 3rd or 4th lower budget season just to make sure they had enough episodes to air re-runs.

Star Trek is the quintessential example of this. When it was first aired it didn't do particularly well and the studio fought to get a 3rd season done just to have enough episodes to put it into syndication. Star Trek didn't become a huge hit until after it started being aired in the after school time slot. Unfortunately by that point all of the sets had been destroyed and they couldn't make any more episodes, leading the short lived animated series and it's eventual revival with the movies and new shows in the 80s.

A long time ago, in the 90s, there was no streaming. Networks needed to fill their daytime slots, so they put "old" episodes of a series there.

A series needed about 100 episodes to air 5-10 episodes per week without airing a duplicate episode every other month.

Also television exploded in the 1980s with satellite TV and cable. The networks needed more content.

Baywatch was briefly the biggest series on earth.

When you make a TV show you have a contract with one network. However after you reach a certain number of episodes, usually 100, your show can then be shown on other networks, which is called syndication. So for example Law & Order ran on NBC, but because it entered syndication reruns are shown on a bunch of channels.

The reason that people want a show to reach syndication is because you can make a lot of money. In fact there are a number of shows where actors make more money from syndication than they did from the show. There's two ways this happens. The first is that the Screen Actors Guild (SAG) has what is called a "residuals schedule," basically an agreement about how much actors get paid for reruns. This is around 6% of your salary per episode. So, for example, the principal cast of Law & Order each make around $200,000 a year from residuals, which is good money.

The second way is that you can have part of your contract say that you are entitled to royalties, which is even more money based on the success of the show. For example the six principal stars of Friends each make around $20 million a year just from reruns and Jerry Seinfeld has made more than $400 million from Seinfeld reruns.

Most TV shows are produced, at least in part, by a television network. Due to the cost of production, the network wants it to be exclusive to them in order to generate ad revenue from the broadcast. After a few seasons, there is a back catalog of episodes that can be sold to other broadcasters for re-airing. This allows the producers to earn more revenue from the program even after no new episodes are being produced.

The way TV worked was that there were a small number of networks who had contracts with the local affiliates. Every city had an ABC, CBS, NBC, and FOX but those designations only really told you what would be on TV during primetime (8pm to ~midnight). For the rest of the day those stations were mostly free to air whatever they wanted. To fill that time they bought the rights to show reruns of older shows.

Unlike the primetime broadcasts which aired on a specific night once a week shows in syndication would typically run every weekday. Because they were running episodes so often the longer the shows run was the more weeks they could get from it without cycling back around to the first episode.

Most affiliates wouldn't touch a show unless it had 100 episodes because shorter shows were not worth reworking their lineups. In response to this demand some shows ran longer in primetime than they otherwise would have to get past that threshold because syndication was highly profitable for the shows producers.