Some electric motors have a binary drive -- they're either on or off. Others are adjustable. There are two types of variable speed motors. One has a potentiometer. You can continually turn it up or down, just like a gas pedal in your car. Another kind of variable speed motor has prefixed settings. For example, it might have low, medium and high, or settings one through 10, but not settings in between. Whether a motor's speed settings are advantageous or not has everything to do with the motor's application.
Many applications only call for one speed. Any more would only add a layer of complexity, more working parts to potentially break down, with no reward. Most table saws, for example, only have one speed. You turn them on, the motor revs up to speed and you start cutting. There just isn't a practical use for a slower or higher speed, and there's no practical means of adjusting the speed while you're using the saw.
Poor Input Method
Some motors with variable speeds have impractical input methods that all but negate many of their advantages. For example, some motors may require the motor to be stopped before it is switched to a different speed, then restarted. You can imagine the impracticality in some applications by imagining having a manual transmission in your car, but no way to shift while you're moving. For some applications, step-speed inputs that require stopping are fine. But the shift method should match the motor's application in order to be advantageous.
A potentiometer allows you to begin at the very lowest speed, then speed up or slow down as you're using the motor. Some potentiometers are like cars and some aren't. Some require persistent pressure, like a car. Most sewing machines, for example, have a potentiometer. If you take the pressure off, it will slow down like a car. Others have a dial control that doesn't have fixed settings, but is static in that you can set it and leave it. Some electric trolling motors work this way, so you don't have to pay attention to them once you speed up or slow down.
Application Specific Settings
Some motors get used in a specific set of ways, where each way has an ideal setting. For example, the motor on a wire feed welder typically has settings from one to 10. If you set up to do a specific kind of welding, it might be advantageous to know that seven, for example, is the right setting, whereas three might be appropriate for another type of welding. This can be a convenient frame of reference.