I think there are a number of different reasons, and EP certainly has a good list. However the reasons are different for different groups of people
I’m not sure exactly how many different categories of people I would say are here, because I believe there are a number of groups that I never interact with. The group I am most familiar with I think I might call the “middle-aged depressed.” Sometimes it is just “the depressed” without any age limits.
These people share certain characteristics besides, or perhaps because of being depressed that make sites like these the perfect solution. Depressed people may crave company, but they can’t get themselves out of the house. Even if they can, they have a lot of trouble connecting with people and making friends, simply because they are convinced no one would like them.
Sites like this are perfect. You can meet people without leaving your house. It’s a built-in social system, and no one can see you, so clothing, tics, and body type can’t turn anyone off. Here, we are all movie stars (well, if we choose to be).
There’s another group I’ve noticed that seems to represent a fair portion of the population here and that is students. They range from high school to grad school, with the predominat population being in college. They have a tendency towards the relationship and human behavior questions, but that is by no means all they ask.
I think that for them, fluther is like the favorite aunt. The one you can ask anything of, and she won’t be phased. She won’t judge. She’ll give you a straight answer (unless she’s in a mood to tease, in which case, she will make jokes).
There’s a group that Bri L just mentioned: people who work from home, and need the connections that you would normally have at a larger workplace.
There are also people who use online sites like this as a built-in support group. Some are very successful because they have a good attitude towards their problems. Others may be successful in getting support in a different way. However the community leaps to support anyone sympathetic who has a problem like cancer or love troubles or suicidal thoughts.
People can fit in more than one of these categories, of course.
I believe there are also hard-nosed techy types who answer all the technical questions, but I’m not sure, since I never go after those questions. In addition, there are a few people who are here purely to have fun, but it is hard to get away with that here, since the mods don’t like open-ended, imprecise questions. They also do not appear to like pure silliness. I think people like this are more likely to be found at other places.
There are also the society mavens—a group that is mainly online for social reasons. They tend to be cliquish once they develop a style, and while they claim to be open to new folks, their style really does not allow for anyone outside their standards to participate. To my knowledge, there are no such groups here, but I have seen them on similar sites.
Anyhoo, that’s what I see. I’d really be interested to find out how other people would categorize the folks here, if that’s appropriate to the question.