It allows you to move power from a live circuit to a dead one by connecting the outlets between them. The most common use I've seen is to easily hook a gas generator to a household circuit during a blackout.

But yes, they're very dangerous and there are less-stupid ways to hook up a generator that won't make tripping over a power cable lethally dangerous or risk burning down the house by bypassing the circuit breaker and overloading the wiring in the walls.

But this one is popular because it can be done very quickly and easily with minimal supplies.