It is my understanding that rocks piled to the top of the tank allows the fish to feel comfortable with using more height of the tank. Their territories are still defined by the foot print of the tank.
I have a 3D background that acts as my big rock pile. There are a lot of nooks and crannies for fish to hide within it. They are very adept at hiding within the background.
I then aquascape the bottom of the tank in a way that appeals more to me while providing enough caves for the fish.