What happens if I turn off the phone for charging?

iPhones automatically turn on when they start charging as far as I know. I just tested it with my own since I was wondering if the behavior was different for wireless charging but no, it was off and as soon as it started charging, it turned back on.

So, I wouldn't advise that you turn off your phone to charge it... as it should turn on if it is receiving a charge (and has at least 5% battery -- otherwise it will display an image of a red low battery on the screen)...

You can certainly charge it when the screen is off (when it's sleeping... but that's different than it being "off"). Hope this helped

Nothing happens except your phone is off. In order to charge it with the phone off, you have to put it on the charger, then turn it off. It will charge about 10 minutes faster. There is no benefit.