"Tossing the ball up" is also know as "flighting the ball" wherein the ball is bowled at more inclined trajectory.
The reason spinners toss the ball up is to induce a false shot from the batsman to get him out. If the ball is bowled with a higher revolution count, then it dips & bounces much earlier than the batsman normally expects. Thus if the batsman is not concentrating hard or is inexperienced, he plays a shot much earlier & likely mishits it to one of the fielders.
The reason a tossed up ball with high rev count dips a lot earlier then it should is basic physics. I don't know the exact term for this but it happens due to pressure difference on two sides.
A right arm off-spinner/leg-spinner spins the ball clockwise in north-east/north-west direction of the pitch. So ball movement on top is opposing the flow of air while ball movement below is aiding the flow of air. So pressure of top of the ball is more than below. So ball moves from higher pressure to lower pressure & hence dips.
The same thing happens in tennis wrt topspin forehand & backhand slice. See my question on Physics.SE
What is the physics behind Roger Federer's & Rafael Nadal's iconic shots