The legend says that once upon a time there was a General Motors. This General Motors, GM for short, had a car and a brand for every need, along the plan developed by the great Alfred Sloan prior to the Second World War. There were Chevrolets for regular folk, Pontiacs for the cautious old people (and, thanks to John Z. Delorean's development of the 1964 GTO, for angry young people as well), Buicks and Oldsmobiles for doctors and successful businessmen, and Cadillacs at the very top, for the most successful men in the land.
It would have stayed that way forever, but one day a mysterious yet important man at GM had a mysterious yet important idea: Executives should drive cars from their own division!
Which leads to every division of GM building their own version of the Cadillac.
Read more: How Fake Luxury Conquered The World
(thanks Bryn for the tip)