It depends on why the admin started their site.
There are multiple others in my niche that are out there (most are Invision). I wanted somewhere to post my content in a format that I was comfortable with. I could have set up a Wordpress blog, but presenting the content, for me, is more familiar in a forum format. If others wanted to join and participate, that's great. I'd be paying the hosting cost no matter what script I was running so why not use something I'm comfortable with?
For most people, if you are going to start a site up, you need a core group with will participate. One person (unless it is a niche not covered, and there are not that many of them now) will have a hard time getting a site off the ground by themself. Even if they can generate good content, most people now are consumers, the actual content generators are few and far between.
I see that regularly with my site. I have several articles posted that are being used by people on another site to change to a different capture based system than what they are using. I have, in each article, an area to discuss that article. But where do they ask me about it? On the other site. I finally commented that if they wanted answers to please join and post them on my site as that was what the discussion area was for and it would also help my site. I mentioned that I only logged onto the other site once or twice a week and they would get a quicker response on my site which I'm on multiple times a day. A couple of them did that... but they started conversations/direct messages instead of posting in the discussion area! 🙄