If you check the discussion forums of forum software such as vBulletin, Xenforo, and PHPBB, you will see a lot of discussions about hosting.
Online communities can take a lot of resources to manage due to the large number of users that are browsing a website at the same time. Every time a page is loaded, many different files and database tables need to be accessed and updated.
Hosting companies that focus solely on hosting discussion forums have a wealth of experience in this area. They know how to configure servers correctly for hosting online communities and are familiar with common hosting problems that arise with forums.