They are for working sheet metal. The plate forms the top of a bench and the stakes engage in the the holes in the plate. Sheet metal is then formed over the stakes much like a blacksmith uses an anvil.
Nice haul. I found one at a flea market similarly tooled up about 10-12 years ago for about $6-700 if I remember right. Was more money than I had to spend at the time, and with no immediate need for one I didn't buy it. Have never seen another all tooled up for sale since. If presented with the same deal now, I'd probably buy it though.