They use the symbols, not only because they are much shorter than typing out the key name, but on many of their international keyboards, they include only the symbol, and have from time to time on the US/UK keyboards. It is the same reason that many times you will see a backwards L with an arrow pointing left to represent the Return/Enter Key, or a large up arrow is shift.
There is also the Box, with a right facing pointy bit and an x in it, that is the traditional PC delete (fn+backspace/delete on laptops). The same symbol flipped is backspace/Mac delete.