Active forums are one of the biggest and easiest ways to add 'Origional Content' to your site. 

Why do you need 'Origional Content'?  In modern times, having a good site isn't enough to get ranked highly in search engines.  If you don't have enough content that is unique and relavant to your site, the search engines will rank your site low... So let your customers build up your content for you with a forum!