I believe those were where the reloads were kept. Although reloading in battle would probably be extraordinarily dangerous.
-Matt
It sure was dangerous if you were an American off Savo Island. The Aoba's were able to savage the Southern force, reload their tubes, then exterminate the Northern force.
On the other hand the Mogami class cruiser Mikuma was destroyed at the battle of Midway because a bomb hit set off the torpedo reloads. According to survivors accounts in the book "Miracle at Midway", Mogami actually received more bomb hits, but because her captain had jettisoned his torpedo reloads she survived the battle.