Those techniques are for armed fights where you cannot afford to let the other person wield their knife/sword freely. (It also shows you how to get rid of people trying to stop you from using your knife.)
As training, they teach you the basics. You can do it without the grabbing, and you can do the grabbing if you want to throw too, it's just far harder.
Because it falls within the realm of "grab my wrist... no, the other wrist..." type martial arts which are simply not effective in a real fight.