I setup a forum a few years back. My brother used to run a successful web hosting service back in the day. There was a blog attached to the main website (not WP, can't remember the name but it was the most popular solution at the time, paid and all) users started commenting and creating a small community right there on the blog, so I suggested installing a forum to my brother.
Lo and behold, the users started using the forums, happy to have a community of their own. It was very nice, with web design sections, html tutorials, general How-Tos - a lot of users had their sites hosted elsewhere, but signed up for the info they found at our forum, and we didn't restrict registration (that proved profitable, as LOTS of users became clients later on).
Those were the times when Plesk was *THE* control panel, as far as I remember (was like 13-14yo at the time). A few years later came the datacenter boom, and unlimited (scammy) web hosting services started popping up everywhere. My bro sold his company to one of the bigger players (still strong today) and moved on to other things. The forums were fused with the new owner's and I stopped frequenting them.
I learned that forums are great when you have an enthusiastic potential community (maybe a niche blog audience, who knows). Just be aware that it takes a lot of combined effort from the owner and the mods to keep things running smoothly - black hats off to mods and admins here, I can only imagine how much dedication and love it takes to run this forum.
tl;dr: Forums are great and profitable, and if you pick the niche/topic right they almost jump start themselves.