There are several motivations in play. A forum is, generally:
- A searchable repository
- A group communication system
- A community
- Simple and easy to use from any browser anywhere
There are many things that can perform one or more of the functions of a forum. A wiki can be used as a repository. A phone conference system can facilitate real-time group communication. A mail list can be all of the first three. Facebook, as it turns out, is a terrible repository, but meets the latter three needs.
Someone might start a forum when they already have a community, such as a mother’s group or anime club, but they notice that there’s information people are sharing again and again, and a forum makes it easy to put those conversations out in the open so that people can be aware of all the conversations going on. Alternately there are a lot of people who would like to participate, but can’t make it to regular physical meetings.
Someone might have an interest, and not finding a community that fits their needs wants to start one. If I were particularly enamored of avocados, to the point where normal garden and cooking groups are getting sick of me going on and on about them, I might start an avocado forum that focuses on the fruit from cultivation through culinary to consumption and even decoration. It may be that it only ever has a few members, or it may grow to quite a large group, but I won’t know until I try, and importantly I won’t care - I’ll have a repository of my own thoughts with a few other people jumping in and helping me or giving me an opportunity to help them.
There are certainly business and monetary reasons to start a forum. If my company sells frobulating whistkers and has some competition, making a forum is a low cost way of engaging forbulators and their suppliers/distributors/servicers which should bring me more business in the long run, compared to the competition which requires you invite a slimy salesperson to soak you in your own office.
I might choose to start a forum as an athlete if I’m planning to participate in a big expensive competition and I need to corral all my sponsors and help in one area to plan, strategize, and get ready for the race.
There are certainly a lot of other forums that are money generators. Certainly a few really big websites started out as not much more than forums. Media websites certainly benefit financially via advertising from engaging users more frequently and for longer periods of time by providing a place for them to gather and kibitz.
Regardless, a forum is only the tool. Like a hammer it can be used for something fun, to help out the community, to make money, and even to harm others. While it will be sad to see, eventually Discourse will be used for some hate group’s community. But it should be much better for the internet and communities in general than a harm, so it’s still a positive tool, all things considered.