At the beginning level, players serve without spin. However, these serves are usually pop-ups that any intermediate (or advanced beginner) can easily attack. As players move up the ranks they learn to serve with spin. But eventually players come full circle and begin serving no-spin again. Yes, this is a shocker to many players, but sometimes nearly half of world-class serves are no spin serves. Why is this?
These no-spin serves aren't beginner-type no-spin serves. At the higher levels, there are two key differences: the serves are very low (whether no-spin or spin), and there's the threat of spin. Since the receiver isn't sure at first whether the serve has spin or not, he can't just assume no-spin, nor can he get into a rhythm against the no-spin since he's getting a variety of serves with spin.
But the key question is why no-spin serves are effective at the higher levels. It would seem that serving with spin is key, and that a no-spin serve wouldn't be much of a threat. But it doesn't work out that way. Here's why. (We're talking about short serves where, given the chance, the second bounce would be over the table. If the serve goes long, then spin or not, the receiver will likely loop it.)
If you serve short backspin, then it is rather easy for a top player to drop it short – the backspin makes this easy as it deadens the ball for you. With the incoming backspin reversing on contact with your racket, it's also easier for the receiver to put backspin on the ball, wither pushing short or long. On the other hand, if you serve no-spin, while it's easier to read the degree of spin (since no-spin always has the same degree of no-spin!), it's not as easy to drop it short. Nor is it as easy to put extra backspin on the ball, since there's no backspin on the incoming ball to reverse – you have to create all the backspin yourself.
If you serve short sidespin or topspin, it's easy to flip. The ball just jumps off the receiver's paddle, and comes out as a topspin, which makes it easy to control the flip. If you serve no-spin, the receiver doesn't have this jump, and has to create all the force of the shot himself, as well as any topspin to control the shot.
So the overall result is that it's harder to drop short or put extra backspin on a push against a no-spin serve short than against a backspin serve, and it's harder to flip aggressively against a no-spin serve than against a sidespin or topspin serve. (A key thing here is serving low – a no-spin serve that's slightly high gets attacked much more easily than a spin serve that's slightly high.)
There's still another advantage of a no-spin serve. While it's tricky trying to serve one spin while convincing the receiver it's another spin, it's probably easier to fake spin and serve no-spin. You simply contact the ball closer to the handle and put it over with a vigorous follow-through, faking spin. This is especially effective if you fake heavy backspin but serve no-spin – watch the opponent push this back and pop it up over and over! And then when you serve heavy backspin again, the opponent is often unsure of the spin, and so puts it right into the net.
So the lesson here is to learn to serve short with spin, and then mix in no-spin serves. You would be surprised how many top players consider the no-spin serve their "go to" serve when it's close.