major 'must pass' bills get all kinds of riders because they're a good chance for horse trading. reb bob wants something on their pet issue and if it gets included as a rider, he'll will support rep carols thing, who will support rep Alice' thing, which will make some allied rep of alice happy and stop blocking yet another thing.

you don't get that done across multiple bills easily, it takes one bill not passing to fuck up the quid pro quo. throw it all in one bill however and it all gets done at once.

its also good for getting Congress critters to buy into the important bill. if someone throws a last minute stink. they're not just delaying the important bill, but pissing off everyone who got their thing added as a rider. at that point it's not just "we need to pass this bill" but "I spent a bunch of effort negotiating riders that might not end up included in the next version of this doesn't pass". plus anyone who got something added as a rider is unlikely to create problems for the main bill.

sometimes it's also just a good chance to deal with small issues that aren't going to make it to a vote on their own. no one's going to object to it being passed, but there's not enough will power to actually get it done. getting it tacked onto a bill that's definitely being passed makes it more feasible.