Repair Disk Permissions: What is it REALLY doing?

I've been told that repairing the disk permissions is a good thing to do about once a month. What I'd like to know is what it's really doing and how important this is.

I just repaired the permissions three times in a row and all three times it found the EXACT same "Permissions differ..." problems.

How can the permissions be getting changed when I'm not even running any apps? It's as if Lion is changing it's own permissions only so that it has something to "repair" when the Repair Disk Permissions button get pushed, however nothing is getting repaired because when it gets run again, the same permission problems are found.

Permissions differ on "System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Support/Rem ote Desktop Message.app/Contents/Resources/pl.lproj/UIAgent.nib"; should be -rw-r--r-- ; they are drw-r--r-- .

Repaired "System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Support/Rem ote Desktop Message.app/Contents/Resources/pl.lproj/UIAgent.nib"