People usually want them so they can run Christmas lights the wrong way, either because theyve already strung them up backwards or they're too dumb to figure out a better solution, but the end result if one end is left unplugged is an enormous amount of power just running loose at the exposed end. Even if you just wanted to plug it into the same outlet as a joke, if you plugged one end and touched the prongs on the other end while in the process of plugging the other side in, you're pretty fucked.

It will either instantly trip the breaker or do absolutely nothing, depending on how it's wired internally.

The real danger comes from when only one end is plugged in, though. With only one end plugged in, the other end becomes a 120V cattle prod.

They're illegal and it's only safe IF the home owner remembers to turn off Mains.