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Doesn't Ctrl + A deselect?
yeah, that's the point of the joke. You'd think that the "default state" should be "select all" -- I want to edit the entire layer, so I should select all of it. But no, "select all" has a bunch of weird obscure behaviour, "select none" is what you want most of the time, even though it gets the shortcut with more keys.
Wait... what does select all and select none do?
Selections are ways to restrict yourself from editing parts of the image. For example, if your select a rectangle, you will only be able to draw on that rectangle, nowhere else. "Select all" and "select none" both allow you to draw on the entire layer. The difference is how some tools such as as Scale, Rotate, Perspective Transform, etc. work.
So, for example, if you Select None, and then use the scale tool to make the layer twice as big, it will scale the pixels contained in the layer, and grow the layer boundaries to accommodate the new pixels. This is what you want most of the time.
If, on the other hand, you Select All, and then use the scale tool, it will cut out all of the pixels into a new Floating Selection, leaving the original layer empty and with the same size. This is a very confusing behavior. Actually, pretty much anything that involves Floating Selection is confusing.