Well, the thing is that GM owns all these companies, like Pontiac, Saturn, Saab, Hummer, Cadillac, and such. However, they don't have full control over all of them, they're just an umbrella. Saab doesn't share much of its cars designs with the other big ones. Pontiac, Chevy and Saturn are in bed with each other, and if one wants a vehicle that another one of the companies already have, they use their design and build their own version of it. Thats why one comes out, then the next, then another.
This strategy sometimes works, because some people may not like Chevy, but might like Pontiac. They put their own flavour into the car. You can by a Cobalt with a 2.2 or 2.0 Super (Turbo now, I believe), but the Pontiac comes with the 2.4, which is better than either of the Cobalt engines. Cadillac has their own version of the Corvette, too. Less powerful, but luxurious.
This is why GM is even surviving, is because of loyalty and cross-breeding their cars. Most Dealerships don't deal with ALL of GM products, either. Saab usually has their own Dealerships. I know we have two 'GM' Dealerships within driving distance here, and one is Pontiac, Buick, GMC. One is Chev, Cadillac. Then there is a Saturn dealer, and a Saab dealer.
Ask the average person if they know all of the companies that belong to the GM Umbrella, and most don't even know how many brands GM actually has.
This is my non-referencing attempt (I know I'll miss one.)
Chevrolet
GMC
Cadillac
Saab
Hummer
Pontiac
Buick
Saturn
Opel
Holden
Vauxhall
Geo (under?)
Daewoo (under?)
Yeah, they own a lot. I must've missed one or two of their brands.