If you want to take it off the grid (versus letting it ring out in silent mode), you could put it into airplane mode.

That's a good idea, but it's also battery conservation and the knowledge that it's also OFF. Certain processes can hang and chew through your battery until you restart, which is also why it's nice to turn it off of a night.

Because when you turn the iPhone off, it's actually off, not asleep. i.e. no processes running, no power to clock chips, etc.