In a classical WC, when flushing, the level of water rises in the bowl with a turbulence which causes a swirling effect and this makes the light materials go to the surface.
This large quantity of water which fills up the bowl is wasted as its only goal is to help drain out the stagnant waste, (approximately 1,5 litre) which is located at the bottom of the bowl.

Result: a lot of water is needed for an often mediocre drainage.