I'd have to actually do it to figure out what occasions I do, but possibly it's when I have a couple of fullscreen apps open or a few windows obscuring the desktop and I want to access something on the desktop without minimising everything out of the way first.
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Holy shit! Win+D doesn't cause you to loose drag and drop!
That I think is what was bugging me, mac at work has expose and show desktop with corner hotspots and the big bonus is you can drag and drop while doing so.
Cheers harvey
I still wish they'd let you hide the icons though, just thought of another scenario, when your opening or saving a document from the desktop, gets harder to find with the extra clutter.