How does it work?
Simple!
When you learn a piece of piano music, you do so by yourself with the help of your teacher. When you play your piano piece you do so by yourself, often to someone else.
Suppose that instead of practising all on your own, someone else was there to help you.
Not just someone else....but a whole orchestra!
And when you need to slow it down to get that difficult bit right, the orchestra slows down too!
...And if you want to practice the left hand or the right hand on their own along with the other part, you can do that too!
Did you forget your music? No worries, it's all here anyway!