Let's say that you're trying to get a ride back to your house.

One friend says that they're not heading in the direction of your house, but that they will drive you home if you pay for all of the gas for them to go there and back.

A group of five friends says that they are going to a house further away, but that at that second house is a group of three friends who will be heading to your house anyway after they get there. So on the way to the second house, you'd split the gas money with five people and on the way from the second house to your house you'd split the gas money with three people.

Even though the second option takes you further, it will be cheaper because it is a route that a lot of other people are taking too.

With planes it's not just the cost of the gas that they save, but all the expenses like paying the crew.