OEM's, Cabs, clean or loaded? kitchen ?'s.

[lightbiter]

oems use up disk space so you have more storage space on your phone to use for ringtones/files/themes and such plus they give you the added ability to if your away from your computer without all your .cab installers when you have to do a hard reset for one reason or another you don't lose all those apps you may have to readjust some settings but you don't have to worry about reinstalling the apps themselves
of course that's just my take on it somebody else might have better reasons