This was always a puzzle to me. Of course, slow bowlers spin the ball, but that seemed like small compensation for bowling slower. After all, the faster the ball is delivered, the less time the batter has to react, and very fast bowlers are nearly always successful so long as they have reasonable aim.
(Faster bowlers are also much scarier than slow ones, and the batter is in greater danger of getting hurt, and hurt badly.)
Yet the very best bowlers in history have actually been slow bowlers - I am thinking of the likes of SF Barnes, Muttiah Murilitharan, and Shane Warne.
What happens with the best 'slow' bowlers is that the energy that would have been used to impart velocity on the ball is instead imparted into spinning it - and a spinning ball does all sorts of things which make it difficult to hit, and even more difficult to hit with control.
Most obviously a cricket ball typically bounces before it gets to the batter, and the ball will 'turn' or move off the straight line according to the direction of its spin. If the ball is spinning clockwise it will go to the right, anticlockwise will go to the left; if it has top spin it will jump forward, back spin will slow its velocity (or sometimes make it skid low).
But that is only half the story, in fact probably less than half the story - because a rapidly spinning ball will swerve in the air even before it bounces. The ball spinning clockwise will swerve to the left - and the swerve will increase as the velocity declines. So a clockwise spinning ball will swerve to the left more and more as it approaches the batter - and then will turn to the right - very tricky to hit!
The situation is reversed for an anticlockwise spinning ball. And forward and back spin have other interesting effects. A forward spinning ball (top spinner) will swerve downwards - so the ball will bounce closer to the bowler (and further away from the batter) than expected. So the ball will come towards the batter, then at the last moment drop further away than he expects, and then bounce rapidly towards him.
The back spinner will look as if it going to bounce in a particular place - but will keep going, and 'float' closer to the batter than expected before it hits the ground - then it might stop and bounce higher than expected, or else skid along the ground.
So, if he can do one or more of these things, a spin bowler can make life very difficult for a batter - especially he can switch between different directions of spin - and also unexpectedly mix-in non-spinning and faster deliveries.
But there is even more! If the spinning ball is angled at about forty-five degrees, the ball will swerve in two planes! For example, if a clockwise spinning ball is angled so the rotation is forward at 45 degrees to the right, then this introduces an element of top-spin. So the ball will do the following: 1.Swerve to the left because of the clockwise rotation, 2 Drop suddenly and land further away from the batter than expected from its early trajectory because of the top spin, 3. Bounce to the right because of the clockwise rotation, and 4. Jump off the pitch toward the batter faster then expected from its velocity because of the top spin!
So a slow bowler can potentially impart very rapid rotation on the ball, control how the ball is angled in the air, control where the ball lands, disguise the various directions and angles of spin so the batter cannot see the changes - and do this reliably many hundreds of times in a row (because slow bowling is less tiring than fast bowling, the spinner can potentially bowl for hour after hour).
To be able to do all this is very difficult and very rare - but IF a bowler can do it, THEN a slow spin bowlers can be most effective and most useful bowler of all.