Why do Dodge and Plymouth make the same cars?

Dodge and Plymouth have made the same cars for years. They are the same company, (DaimlerChrysler), different 'brands'. Cars like the Dodge Daytona and Plymouth Sundance were the same body style with different accessory packages.

Recently, with the advent of the 'Neon' in the early 1990's, they stopped bothering to call the cars different names, but still offered different accessory packages depending on whether it was a Dodge Neon or a Plymouth Neon.

In any case, I'm sure it has to do with the logistics of one company fabricating many different models under several different brand names. It was a way to minimize assembly-line work and space and still keep separate images for the two brands.