As a recent member I'll share my pov.
1. Strategy game single player. A complex game, with many features, strategies, different ways to play it... A forum is a perfect place to learn new tactics, details, mods, etc from other people. As said, the user profile is someone who wants to be informed. Can you imagine a forum for, let's say, counterstrike? I like that game, but I would never go to a CS forum. No need to.
2. The community. The people wanting to learn about the game, that find many people helping and sharing information. That encouraged me to be an active member too, to receive help and to share when I'm able to.
3. The developers. Showing interest in many different aspects, like this post, loving to improve the game, asking for feedback to do so, answering too (I really like to see Kael posting that "Fixed, thanks" ...). That makes the line between user and developer fades away...
So, mixing it all, you get a forum where you can talk, discuss, ask and receive answers, share ideas, and Devs you are here checking wich ones may be good to quickly implement them (I'm wishing to try those new Juggernauts).
And as stated before, it seems that there is a Despair spell for trolls...they just disappear