1 - I think you could use it only when making a big game (lots of graphics) - means, I've never needed to use it.
2 - Loading/unloading happens automatically by your graphic card. When you switch layouts, graphics from previous layout is still in the vram and stays there in case it's needed again. New graphics from new layout pushes old one out if there is not enough space anymore (depends on the size of vram). To answer your question, unloading happens automatically, but not by ending a layout. If you really wanted to empty the memory, you'd have to use the action you mentioned.