If a receptacle is added from a switch box, will the receptacle only work when the switch is off?
When the switch is open, then a low power device (phone charger for example) would work - until someone changes the 100W incandescent bulb for a low power LED...

Once the switch is closed that low power device will get the voltage generated across the contacts due to the resistance of the contacts and the current flowing, which will be too low for the device to work.

Putting a bulb into a circuit to help fault find is also a good trick...

On a side note, when I was about 5 years old, I was going round turning off all the sockets in the house (UK which have switches on sockets). Mum asked me why... I said " so the electricity does not run out onto the floor...". Now Mum is not an electrician but did smile, Dad however, is an electrician and creased up with laughter and started to teach me about wires, lights and circuits...