Interesting question. Many reasons really. Friends can have enough of their own problems to deal with so don't need anything else. Men, and yes I'm being sexist here, often have trouble discussing problems with their (male) friends. STW can offer anonymity.
This is a broad church and everyone is dealing with difficulties of some description. I personally have received a lot of advice/support in the past when I strugged big-time with a situation.
Families are often the cause of the problem. An outsider can look at things clearly and without passing judgement.
Am always happy to listen to people, whether face-to-face or by e-mail. Nobody should feel alone and I genuinely mean that.