the foam is put underneath to distribute pressure points for unlevel stands, or not so flat aquarium plastic frames. The foam will compress more so to distribute the pressure at one point, "equally" supporting everywhere.

This of course makes sense, but some, including tank manufacturers don't recommend it (Oceanic? I think they say NOT to do it), it may be an old wives tale.

That being said, I have white foam under my tank. I've previously used carpet underlay on a 75gal.

Live rock is actually "lighter" for the space. Water is heavier and the rock actually displaces little rock and only appears to have a lot of volume..