The Norinco AR rifle [also the Dominion Arms DA556] has a commercial sized buffer tube/receiver extension. A lot of the Magpul stocks are _milspec_ which has a slightly different diameter buffer tube and related parts. Why this should be is not known to me, and seems kinda strange, to tell you the truth....the differences are tiny. Apparently it's possible to install a milspec tube on the Norinco receiver, however the tube kits cost upward of $100, and when paired the price of a new stock, don't really seem cost effective. If you're shopping for a new stock, be sure to look for _commercial_ stock hardware, not milspec.
Advantages? Weight and length...build strength; in some, the design guards the extension latch. In others, the buttstock has a storage compartment for batteries, cleaning gear and etc. In some cases they simply look cooler or fit tighter so they don't rattle on the buffer tube. The AR appears to be the Barbie of guns...you can spend more on parts to 'dress her up' than on the basic gun itself.