Not every Couchsurfer is on an extended trip. Some are just in town for a week or so and prefer surfing with hosts to staying alone at a hostel or B&B. Some are coming here to work or start Uni and need a temporary place to crash while they look for accommodation. The hosts I’ve met have for the most part really enjoyed showing people around and having a good time with them, though of course there are some who don’t have the time or inclination to do that. The reason for not being able to plan too far in advance has already been addressed, so I won’t go into that except to agree with what the others have said.
And, as Parkhead pointed out, students and other young people with fewer responsibilities at home do tend to be overrepresented, but they are not the only ones using this system by a long shot. There are quite a few older folk with long-term jobs who Couchsurf when they go on vacation and host when they’re at home. In addition to finding brief temporary lodging while on a trip, Couchsurfers also seek each other out to learn more about an area they’re moving into on a more permanent basis. A lot of the posts I see on the local forum are asking about the best places to live in town, how to find a flat, how to get around, what the job market is like, who’s up for having a drink or going out to a show, etc. It’s a great way to meet new people when you’re a stranger in a strange land, as I am. Some of those people will just be passing through, but a lot of them are local.
The local group has also spawned some offshoots such as the Kitchen Surfing group I mentioned earlier, which cooks and eats dinner together in someone’s home, and a photography group that meets to learn more about the subject and share photos and techniques with each other. There’s also a “free stuff” group for people who are getting rid of things and want them to go to a good home. This is particularly useful for students who are leaving uni and have a lot of furniture, appliances, etc. that they don’t want to drag home with them but don’t want to chuck in the bin either.