The styrofoam keeps your hand from freezing while you rub on the ice. Ice for about 5 minutes or until you're frozen. Repeat before you go to sleep. If you don't freeze a cup of water you will blow off icing a sore spot when you shou ld have. It is inc venient to get out ice cubes and put them into something after your run.
Ice is the miracle drug of choice.
Not icing can lead to bad pain. If you have a pain creep up during a run and it seems to be getting worse the more you run stop. Do not try to get in even one more mile of your run. Pains that come on during runs can turn into serious injuries if you run throug pain. Stop and walk, or call for a ride.