Holds your guitar when you can't
This serves a very specific purpose: if you play a guitar-like instrument at folk music sessions in pubs, then you don’t normally have space for a conventional guitar stand, and putting your instrument in its case when you want to get a drink is awkward. This device clamps onto the edge of a table and holds the guitar securely upright by the neck.
I’ve had one for a couple of years, and used it both in sessions and on stage — it’s also easier to carry around than a guitar stand if you know you’ve got a table to clip it on to. I wish they did a “drop” version for short-scale instruments, though…