1- Helping others with your experience.
2- Group intelligence phenomenon.
3- Healthy speaking about yourself and your feelings.
4- Socializing, in case you were a nerd.
5- Recieving feedback on your opinions. (well, disregarding my blog, there is a lot of blogs with a lot of comments in it)
6- Sharing ideas with interested people.
7- Personality representation.
8- Hobby and a "social" activity. (maybe you can even write it in your CV)
9- Timewasting if you have nothing to do, or if you are just bored.
10- (this one is left for the reader, com'on; use your imagination.. ).
(note : please just 1 comment, I want to see a comment before I die, pleeeeease!!)