Nominally you can use it to plug a generators output into a household circuit, which will provide power to that circuit in cases of a blackout, saving you from needing to unplug everything critical and daisy chain 10 multiplugs to the generator.

It could also be used to connect two seperate household circuits together, if only of them is actually live for whatever reason.

In reality you shouldn’t his at all, ever. Just daisy chain the extension cords. If you forgot to isolate the circuit by flipping the main breaker (easy to do if there’s no power anyway, because of a blackout), and then the grid comes back on, your generator is gonna have a real bad time. And then there’s obviosuly the electric shock risk of using something like this.