Second one: battery life.
If you leave the unit on, even if it's in a sleepy state, it still takes some power. Not the screen though, but the processor needs power to be alive. Turning it off means it uses absolutely no power (other than maybe an internal clock?), and only the self-discharge of the battery occurs. I think someone said that turning the unit off and on again takes as much battery as ~15 minutes of idle time...