Save on gas, stay in a group easier...trying to be environmentally concious by not taking 5 vehicles for a group of 5 but rather one vehicle that the 5 can fit in, maybe some people in this group don't have a car so they ride along....there are various reasons people car-pool, though not everyone is good to car-pool with, like the person who was wait and expected you to wait around a bunch.

I think it can be a useful thing, though not so much if people end up being totally unreliable with it. I end up carpooling as I do not drive, but I respect the persons vehicle and am typically willing to offer gas money(when I am flat broke not so much but then I don't expect them to go out of their way then its if they help me with a ride cool, if not then there are buses or walking) but also usually I end up carpooling with people who also want to hang out with me.