With hobby mini CNC machines you get simple and very cheap clamps that are not easy to use.
It is hard to fasten it because screw turns along with nut, and as screw head is in T-slot of bead it is not easy reachable to block turning.
This is small rectangle peace you can attach to screw head that will block it turning within T-slot.
It makes using clamps much better experience.
You may make it from mdf, wood, plastic, metal, whatever you like.