For one, it blocks stray light coming in making the screen image brighter.
2- is the adjustable magnifyer/diopter. The image is a bit enlarged and you can correct for you eyes by twisting the lens ontop.
There is a metered version that has a spot sensor you can move around to any place in the image.
Once you try it you'll never want to use a collapsable ever again.
KEH has em occasionally, if you see it grab it, they go fast.