With Ballet Class you can control the speed without affecting the pitch, which other devices like CD players usually affect. The effect of pitch shift is that the music goes out of tune, it may even sound funny if the speed change causing it is significant.
Besides, you can save the tune with a modified speed as a new tune, so that you don't have to find out the correct speed again and again. You can even create different copies, each with a specific speed (say, for different courses).
Moreover, speed control is not everything to it: you can select how many bars you want the tune to last just by tapping one button. No edit of any sort is needed. And you can save this kind of changes too.