In addition to the reasons already listed here... a forum will help your websites search engine rankings - especially if you can attatch it to the URL for your site. Forums are rich in organic content and in addition to repeat vistitors, it'll improve your search engine rankings and bring traffic via Google for key terms you wouldn't be optimized for without it. In addition, a simple banner rotation can bring in additional advertising revenue; you can also add Google Adsense to your forum for further monetization. Best & Take Care!