The dimples serve several functions. They trap small amounts of liquid and thus promote even cooking, they make the pan rigid, and they prevent warping. They're also a nostalgic reminder of the days when paella pans were hand hammered. Some people claim that the dimples keep the rice from sticking to the pan, but I'm (Sarah Jay) not convinced. For one thing, rice sticking to the bottom of the pan is not something you want to avoid, since it helps foster one of the most succulent and seductive aspects of paella, something called socarrat ( the crusty bottom layer of rice)