It all depends on how much traffic a site gets. With the usual traffic on a forum serving less than 20-50 members, most shared web hosts won't have an issue serving whatever is being requested. Add a couple hundred users to that (or more), and small issues that increase a slight performance requirement on the web server turn into big issues since it is being repeated exponentially. Running a free utility like I linked above helps people to fix those small issues before they become large issues.