I am no expert but I believe car companies like to have multiple brands. It allows them to increase market share. For example, if you go shopping for a car you may find yourself choosing between a Skoda, a VW, a Ford and an Audi. You think you are choosing between 4 companies but in fact three of the four belong to the same company (VW own Audi as well). You do of course have choice between models still, but if you look hard the similarities between e.g. an Audi A3, a VW Golf and the skoda equivelant are vast (chasis, wing mirrors, gearboxes, etc, etc, etc).