What, exactly, is the purpose of the *.icloud.com ID?

Ok, so this is probably an elementary question, but one I have yet to figure out. Apple, in their infinite wisdom, has rolled out "iCloud" id's - So I now have my original gmail ID and an iCloud ID. The two seem to be equally interchangeable when logging into apple services. My question is, why then is an iCloud ID necessary? What purpose does it serve that my gmail ID logging in does not?

The iCloud ID is more for your iCloud email purposes. You cannot use JohnDoe at gmail as an iCloud email account.