caches locally save resources to be pulled later in a quicker manner, so the client wont have to re-download them. MMOs usually require you to download most of the resources, which you could consider a local cache, I suppose.

You sometimes need to clear a cache to lower the size, refresh it if you will. A smaller sized cache means a smaller find/use rate for cached resources. Sometimes these things can be corrupt, and need to be cleared to solve the cache 'errors'.