The biobale doesn't have any living things on it, that might stink up when exposed to air.
I figured if there was life on the rock, and aired out, it might wreak like hell. Since biobale is just plastic shavings, it only provides a home for the bacteria, which dont stink too badly.
As I undertand it, biobale also acts to create lots of surface area with the water, enhancing oxygen entrainment. That is versus leaving the space empty.