Why do people use online communities?
People use communities to connect with the same minded people, usually to solve some problems together or share some interests. If people can get together offline effectively, they will definitely do that. Think of church communities. They have a physical place and a certain time to meet. But not all communities have this kind of luxury. Let’s say, what about people with some disorder located all around the country in different timezones. How can they get together? The only answer is on an online forum.
So people use online communities for the same reasons why they attend the offline ones, but when the offline collaboration is inefficient.