[dellcybpwr]

What are use cases for power inlets?

Also, are they hot if nothing is plugged in?

[TheCoelacanth]

Inlets are for putting power into something, just like an outlet is for getting power out.

They are reversed from an outlet because don't ever want the hot part to be exposed.

[bluGill]

Main use is a backup generator. Your electrical panel is supposed to have an interlock so you can get power from either than inlet, or from the power company but never both. When the power goes up you plug your generator in, flip a switch and your house has power. Legal and fairly common.