You can see, for example, why the iOS UI puts rarely-used or navigation buttons on the top of the screen. Since most of our usage is right around the middle, developers want to make sure that buttons that require any traveling also have a suitable importance of function and thus are prominently displayed. And you can see the simplicity of certain apps, too, as Angry Birds and the YouTube app only have a few controls, while a game like Fieldrunners can get relatively complex.
Interesting stuff, all divined from smudges we wipe off daily. I don’t think Apple provides heatmaps like these to developers, but maybe some coders trying to figure out how their UI is used can check the fingerprints after a usage session and see what they learn.