Benefits to having a forum on a website
Visitors who are engaged tend to stay on your site longer and visit more often. Forums give your site constantly updating information, with no additional work on your part. In some cases, if you set up a support forum, your users will answer each other's questions, saving valuable time for your support staff.
Forums can be restricted so that only members of your site, or people who have a password, are able to view them. This can allow you to require that members sign up before posting to the forums, which allows you to build an email list, and makes moderating the forums easier.