I understand there is a need for publishers in physical media such as Xbox DVDs who handle the physical production and marketing. But why is there a distinctio

I understand there is a need for publishers in physical media such as Xbox DVDs who handle the physical production and marketing. But why is there a distinction between developers (Rovio Labs (Angry Birds), ZeptoLab (Cut the Rope)) and publishers (Chillingo) on the iPhone? What purpose do publishers serve on mobile device distribution?