When you use it to roll out fondant. A fondant sheeter is nice, but much MUCH more expensive, and it needs electricity.
This can go anywhere, takes up minimal storage space, and does not rely on external forces ton work. Just crank and go.
It's too small to roll out fondant to place over a cake, but perfect for making swags and other fondant decorations.
A must-have for any cake decorator (right after a spatula, turntable and rolling pin).
Starting at $30 new, most good ones run $80-$100.
Want a tip? Keep it clean and oiled (dried fondant is EXTREMELY hard to remove).