Prior to the mid 1930's tanks were hung on the wall. You'll also notice the water spot in the bowl is smaller, since less water is needed to operate the toilet efficiently. Traditional toilet flush is gravity-fed. All the water stored in the tank rushes down, forcing everything out through a built-in pipe called the trap. The trap curves down, then up like an elbow.
Water fills it to keep sewer gases from entering the house.
There is an alternative flush type called pressure-assisted. A vessel inside the tank holds trapped, compressed air. It acts like a spring to shoot the water forward at a high rate of speed. A large percentage of the 1.6 gallons of water used is delivered directly to a jet at the bottom of the bowl.