I broadly look at forums as archives of human knowledge. No matter what the topic or genre, people who connect with a particular subject will seek knowledge from more experienced members of that particular subject/industry.
These answers provide real value that is difficult to quantify unless you're in the user's particular situation. The knowledge a user receives could save them time, embarrassment, money etc. It could also build social status in the user's real life as they could be perceived as more capable and experienced than they actually are. It could give them confidence to take on bigger projects with the knowledge that they have a place to find answers/support consistently.
And the list could go on... but most importantly the cost of admission is Free. Specific advice from a qualified individuals usually costs money or is hard to find at the very least and a forum therefore offers a convenient alternative.
Say for example that i'm building a DIY home renovation forum. What impact could quality advice have on a member who wants to remodel their bathroom but has no experience? How does the user feel when they receive quality advice from a seasoned member giving them a starting point, an experienced perspective and possibly continued support? What happens if they succeed in the project?
The answer is they come back...again and again.
Another equally competing motivator is simply a feeling of inclusion in the form of community with the people who share the same passion or interest. Hence a strong visual identify for your brand can represent this situation and attract members like bees to the nectar.
Of course, you have to make your presence known otherwise you won't be considered. Consider this:
Why are flowers bright and colourful?
They are like advertisement signs for pollinators. In order for plants to entice pollinators, they must first offer their favorite foods: nectar and protein. Since most pollinators fly, the colors of a flower must attract them, therefore, the brighter the flower, the more likely it will be visited.