Because sometimes the best therapy one can get is a catharsis of feelings through open and unjudgemental communication.

Also, being able to talk like that about things of that weight mean that you've begun to understand their circumstances, and deal with them better than you have before in your earlier life.

Also, the internet is a strange place. Sometimes people begin to treat the internet like an open journal, due to it's text nature, and thus feel more open to communicate with strangers things they may never have told some of their closest friends.