If a Subversion command cannot complete successfully, perhaps due to server problems, your working copy can be left in an inconsistent state. In that case you need to use→ on the folder. It is a good idea to do this at the top level of the working copy.
Cleanup has another useful side effect. If a file date changes but its content doesn't, Subversion cannot tell whether it has really changed except by doing a byte-by-byte comparison with the pristine copy. If you have a lot of files in this state it makes acquiring status very slow, which will make many dialogs slow to respond. Executing a Cleanup on your working copy will repair these “broken” timestamps and restore status checks to full speed.
Some earlier releases of Subversion were affected by a bug which caused timestamp mismatch when you check out with the Use commit timestamps option checked. Use the Cleanup command to speed up these working copies.