It's a setting used to try and balance the fastest players (92 and up speed ratings) and the slowest players (76 and under speed ratings). Any player with a speed between 80 and 90 aren't really affected, I've also noticed it helps keep players feet on the ground, they don't glide or slide as much. Balancing the speeds creates more Realism. It limits outrageous recurring big plays and makes them more rare and earned. Most NFL players are only a few steps in each others speed range. Yes linemen are slower but RBs, WRs, CBs are all around the same speed of each other. With my speed parity scale setting at 90, elite speeds are more noticeable and big plays are earned so you'd need to really pick a play with a purpose, you need to look at match ups and take advantage of a weakness.