Bombing

They mentioned in the Iwo Jima film that very little was actually accomplished by bombing the island dispite severall raids both before and durring the invasion. It leads to an interesting discussion on the reasoning behind doing it in the first place. I heard that it was not uncommon for commanders to fire heavy amounts of boms/artilery at a possition for no other reason that to lead GIs on the ground to feel more confident about the possition they were preparing to take. I understand that the Japanese had sufficient time and knowledge of the island in order to build defence strongholds throughout. And it makes sence that givin their underground nature, the recon photos that US commanders had to work with may not have suggested how well the Japanese were dug in.
But it leaves a question in the air of what was the real motivation behind dropping so many shells on the island when it didnt have that much effect. Did the commanders just assume that the Japanese were there even though they didnt have exact locations of troops? Did commanders not even care about taking out possitions as long as troops thought they would have a fighting chance?