"Dancers need a certain amount of speed so they can keep their balance" - you mean momentum. If the music is too fast you are constantly off balance and have to keep moving on to the next step to prevent falling over due to the momentum. If the music is slowed down you can maintain a more upright posture and control your speed more, slowing your rate of travel (not number of steps per minute) or speeding up as the dance demands. This puts more dynamic into your movements. You also have more time to concentrate on what your feet are actually doing and incorporate stepping into the dance.