As a lazy man at heart, I enjoyed the thought of getting credit for my computers' work. Other than interest in space/extraterrestrials and science in general, I have no compelling story about why I run SETI@home.
Thoughts about SETI and SETI@home
It's most likely a question of when extraterrestrial life will be found (and a related question of how one wants to define such life). I think humans ought to expend reasonable resources (more than currently expended) in an attempt to find such life.