Foams are more of a tuning aid to try and squeeze out that extra little advantage over the next guy. A harder foam will allow the tire to maintain it's shape. Under acceleration, we've all seen our tires balloon. A harder foam will make an effort to force the tire to maintain it's shape which in turn will give you a greater contact patch with the ground. Also, a harder foam will reduce sidewall flex when turning.

Softer foams would probably be better suited for loose track with lots of bumps. I don't know that they make a huge difference though.