For the user it just makes it easier to see who is better for your system (see my example above a few posts).
For the CPU it allows them to target the correct players for their system and more importantly to set their depth chart more appropriately. They will put a 60 spd QB ahead of a 90 spd QB in an option setup if that 60 spd is 1 point higher on the overall even though he is truly a horrible man for that position.