Go back to my pictures and see how it has a plate sticking off the side for mounting to a flat surface?
This is different than a vise that would normally just mount right onto the top of a bench.
This is so it sticks out from the bench, and then there's the "leg", which is why these are often called a "leg vise".
This leg transfers any energy from hammer blows to the ground, rather than shaking up everything else around.
Do searches for "leg vise" in your searches as well.
Blacksmiths use these often to grab their work pieces for bending and twisting, etc. since so much of their work involves such things.