Layer's are simply a way of making it very easy to hide and show various groups, without having to move them, etc. So a single click, for example, gets rid of the front baffle, and then brings it back, without having to worry that I (probably) would slightly messup the dimensions while moving things around
PM if my explanation made no sense, I can show you what I mean!
Love the model, should be a real help to future DIYers! If you get bored, do the OB-7 Plus!