aspen pretty much hit the nail on the head as he covered the major benefits. I think one of the key benefits of running a community is the ability to receive instant feedback from your visitors and to interact with them in ways you can't otherwise.
i think apart from the benefits of traffic within your forum, you are generally more likely to build more traffic to your parent site, as you have the loyalty factor, people are also more likely to refer if they feel they have some involvment with the site...
I think that is one of the major benefits of running a community, tho i was conidering running a community for my next project ive decided against it because of looking at competing sites and the amount of flaming that goes on in those forums.
The Sitepoint forums are fairly peaceful...on the other side of the spectrum Rage Against the Machine boards. The official one got closed down because of all the contraversial talk, it was even on the radio.
I think that the act of creating a community in and of itself is very beneficial in a more qualitative sense. Giving people the opportunity to discuss things in a way they were never able to before is very powerful IMHO. Before the Gymn.ca forum was created people interested in Canadian Gymnastics never had many opportunities to talk to each other. We created a community through the messageboard that goes far beyond generating hits for the site. Actually, I could care less how many extra hits the main site gets because of the forum. The community aspect makes that site better and more meaningful to the Canadian Gymnastics community in general. The ability to participate is meaningful to both our visitors and ourselves.
So, I guess you could say that community can really improve the quality of your site. That does relate back to loyalty, but it's more than that. When you create a community you involve people, they become more than just visitors, kind of like a part of your team.