Most toilets in house have tanks. You press the lever, the tank empties, flushing the toilet, the tank refills for next time. Many commercial buildings have toilets without tanks, just a bowl mounted on the wall. (Side bonus, it’s a whole lot easier to clean the floor when the bowl is mounted on the wall.)
Why don’t homes have tankless toilets? I seem to have to replace the flapper/float assemblies in toilets every few years. Something starts sticking and my toilets run otherwise. If I could dispense with the tanks this wouldn’t be a problem.
In areas with low water pressure, I know the tank provides the volume/pressure to force everything down the drain, but my water pressure is fine. Our house has a regulator to decrease the pressure before it enters the pipes.