Not sure why they use post vises, aside from the parallel piece that Jim brought up, those vises have exposed leadscrews, Ive always been under the assumption that vises with covered leadscrews are stouter, and it also keeps swarf/shavings/crap out of the leadscrew/leadscrew nut.

Also notice that they dont have any swivel vises. I suspect that we may be looking at only one part of the operation, with very specific jobs. Very likely theres another part of the shop with bench vises.

The only benefit I see here is that the fixed jaw is far away from the bench, allowing one to hang long parts from the vise and ensuring that it doesnt hit the work bench.