|Donations are used to fund Blender development. The Blender movie projects are, in a sense, a side effect. They take new features for a test drive, to check what works and what does not. They also demonstrate what Blender can do. Each movie project has a theme or area of focus - for example, in Big Buck Bunny it was fur, for the latest, Tears of Steel, it was visual effects.
If you are interested in contributing to movie projects, you can probably find a few at BlenderNation.