Hi, I have written a new library that allows layering in sketches.  By using the API, the developer can create layers (like in Photoshop, etc.) that are drawn independently of one another and layered on top of each other in the sketch.

Everything is documented at http://nootropicdesign.com/processing-layers/ and there are some interesting demonstrations of its capability.

I set up a forum for discussion here:
http://nootropicdesign.com/forum/index.php#2

I'd like this library to be added to the Libraries page at http://processing.org/reference/libraries/index.html but I'm not quite sure what category this fits into!

I hope some of you find this library useful and look forward to your feedback.
-Michael

Good questions.  You understand the main benefit of not having to draw everything every frame.   This allows for animation which requires erasing everything between frames.
The laser example takes layering to an extreme by having a different layer for each component of the sketch.  You certainly would not need that much separation.  And the simple animation sketch with the blue circles does not need layering to create.  Another benefit, however, is an object-oriented approach for drawing a sketch.  Each object is responsible for drawing itself with it's own draw() method, without regard to anything else that is happening in the sketch.  I think this opens up a lot of possibility as opposed to having one large draw() method that is responsible for drawing everything and keeping track of all data structures.

Does that help?