IPhone Game “Cut the Rope” Sells 1 Million Copies in 10 Days

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?