Saves money and most consumers still belive there is a difference.
Back in the day GMC was created to give Pontiac, Cadillac, Oldsmobile and Buick dealers something to sell. Since none of them built trucks and it was too costly to add the entire Chevy line just to be able to sell a pickup truck to a Buick customer GM decided to create the GMC consumer line of trucks. It would have been way to costly to build all new trucks from the ground up so they just put a new grill on a Chevy added a few interior changes and called it a GMC.
Same thing with the Escalade, When the SUV craze hit big Cadillac wanted in on the action so they dressed up a Suburban and called it a Cadillac. When they realized how many Escalades they were selling and much profit they could make by dressing up the cheap to build Suburban in fancy leather and chrome wheels Chevy wanted in on the big profits too. So they created the Denali for GMC and added to the option list for the Suburban.
It's all about easy profits.