Expose moves them all out of the way, but only temporarily. As soon as you select something, they all come back. Hide All moves hides them all to the Dock and they stay there until explicitly called back by a mouse click on the Dock icon. If you have lots of open windows, this can be a quick way to send them all away and focus on something else.
Of course I agree with you yogi - Spaces is another entirely different solution to a similar problem. I used YouControl Desktops right now for this very purpose - multi desktop capability. When I want a clean desktop, I can either Hide All or simply move to an unused desktop. In practise, I use 4 desktops, and keep the fourth one free for just this sort of thing.