Why do people join online communities?
Tl;dr; People join online communities because they see benefits in doing so. That is it.
People join online communities mostly because of the mission of the communities and then stay because of the culture and other users. To understand that let’s dig a bit deeper and think about motivators. I would say that there are two types of motivators. Extrinsic and intrinsic motivators.
External motivators are based on material or intangible incentives and rewards, like gift cards, or extra privileges in the community. Intrinsic motivators are based on a sense of satisfaction, pleasure derived from doing an activity itself. Intrinsic motivations can be personal (a sense of autonomy, the ability to decide what, when and how to do things, and a sense of competence) or social (the ability to connect with other members of the community and being able to share your work with peers and help them).
So, the primary reasons why people are active in a community are personal or social intrinsic motivations. But before being active they need to make a decision to join a community. Here is where the mission of a community plays the key role. Because, basically, a community is a group of people connected by a shared goal, an interest or a purpose and the shared goal, interest and purpose are exactly what the mission of a community is about.