Part of Apple's design philosophy is to keep things simple: The company believes that users shouldn't need instructions on how to operate its gadgets, it should just be obvious.
One way Apple has achieved that is by using a series of longstanding icons that we've all come to recognize. These symbols are so familiar, we don't need to be told what to do.
Apple has mostly followed a "skeuomorphic design" strategy: its symbols look like literal representations of the objects they represent, so the Newsstand app appears to have wooden shelves, just like an actual newsstand. (And at one point there was a sort of civil war inside the company over whether this skeuomorphism had gone too far.)
As such, a lot of Apple's icons have decidedly vintage, non-digital origins. We bet you don't know what the curly square on the command key started out as, for example ...