Spin is desirable because it allows the movement of the ball to be less predictable which leads to points for you.

In addition to making it harder for your opponent to predict what the ball is going to do, spin is also useful for controlling the direction of the ball and for increasing your margin for error. For example, on the second serve, most players will put more spin on the ball by brushing up behind it to hit what's called a kick serve. It is called a kick serve, because when it is hit well, it will kick when it bounces, making it more difficult to return. The extra spin enables the server to get more clearance above the net, and then the ball dips down into the service box, almost straight down. This gives the server more margin for error in getting the serve in. Another type of spin is the slice, where the server hits the ball more on the side rather than behind it. This spin causes the serve to curve out wide and is very effective when trying to pull the receiver off the court. The slice serve can also be used to hit a serve that curves into the receivers body, making it hard for him to get out of the way. For body serves, the server aims more for the middle of the service box. Spin enables the server to swing harder and still get the ball in. If the top servers didn't have any spin on the ball, the ball would never go in at those speeds.

When you put more spin on the ball you can direct the ball to where you want it to go better. I t also gives you a more margin for error , like going over the net but not going out long, it gives you a more effective and consistant serve.