OK, so I watched another of those internet videos that does not have a proper introduction. I was doing something else at the same time so I guess it does not count as a total waste of my time.

But even at the end there is no clue as to just what it is for. I can see that, with the help of another clamp of a particular type, it can clamp one or two boards on top of a larger board. But WHY? Just what is it for?