just as stated before..
it allows the weight to deload into the board and into your body to maintain form, function, and TIGHTNESS to allow you to work a particular range of motion (especially for shirted guys.. i don't believe this has any application to raw lifters anymore) you might be weak in where as when you use pins or into a rack the weight deloads into the apparatus and not into the lifter making it less functional and DANGEROUS...