Ever notice how computers have a lot of options, like: reboot, sleep, hibernate, turn-off? A lot of people don't know or care about the difference between those commands, while they interrupt stuff that the computer may be doing in the background (like downloads or polling for new messages). Most mobile devices can go into a low power mode that will continue with those background activities with the press of a button, which is why I stuck "turn it off" in quotes. Of course you can actually turn most of those devices off if you tried, but I'd suggest that most people never even bother.