The clip can be found on the back of many pocketknives. It is used to secure the knife for quick access without losing it. Most people use it to clip the knife to the inside of their pants pocket.
It is important to remember though that it is not a belt clip. None of the clips are intended to attach a knife to the belt. It is much too dangerous, because an unprotected knife might be opened accidentally through outside influences. In addition, clipping the knife to the belt puts undue strain on the clip, which might tear the threads of the clip screws.