My house is an older one, 1950's. So the wiring was designed for a radio and a couple of lights. Not the massive amount of gadgets you can plug in nowadays. We also had a scary wall heater.
It was wall mounted with 220 volt oven style heating element, with large mesh that a finger could access. The mesh would heat up to the point of burning flesh. As such we disabled them early on.
During the winter our non-central heated house gets rather chili. Connor has been complaining about the cold in his room, so you know it must be damn cold. I decided to add a wall outlet using the wiring.
So I took apart the wall heater and surveyed the wiring. There were two sets of wires. I called my Pop, who knows more about this stuff then most humans on the planet. He made a few suggestions and tests. Sure enough these wires powered this heater then went off to the bathroom.
I had to back trace the wires, change out the breakers and run the white to neutral to power down the voltage. I also ran a ground wire to the box, so it would be that much better.
Wiring a wall socket then attaching the box to the wall is definitely the way to go!