This is a threaded part that mates to many common bottles. If, like me, you have a neurosis that means you cannot stand wasting that last little bit of liquid in a bottle of shampoo or soap, you can use this to easily transfer the remaining contents of the old bottle into the new one.

Mainly I wanted to try printing something threaded without supports, and it worked well. A surprising number of bottles of this size seem to work with it.