[ars]

The main thing publishers do is advertising. The second main thing is getting the product into channels (not an issue here of course).

And sometimes publishers will actually fund a project, usually in the form of advances.

I don't know Angry Birds specifically though.

[rhizome]

I haven't read about the deal, but I'd imagine EA also got rights to the game for other platforms, and that that is the primary value here. For app store stuff it's probably more a case of free advertising.