Everyone has different reasons, these are 3 main reasons why I do distributed computing.
1) The science - by doing this you are actively contributing to help find cures for diseases, space research, predicting climate change etc. Some people give money to charity, I prefer this as it's hands on and I also enjoy the other 2 reasons.
2) Community - you won't find a friendlier part of the forum than here, there is some great banter and competitiveness as we try to move up the ranks within the team and banding together as a whole to take on other teams and compete in events.
3) The gear - There are very few things that actually need you to build a top of the line system, by crunching you really are making the most of your machine - how many people on here have i7s to browse the web and play the odd game? With DC the challenge is to build the most cost effective powerful crunching machine that is stable 24/7, even under constant load. I love building rigs and this gives me a good reason too.
edit: It can be expensive as you want it to be, some people just use there spare cpu cycles while they've got the machine on for other tasks which they would have done anyway. The more enthusiastic among us will build dedicated rigs, but there is no minimum level, anyone can contribute any amount.