Alot of places your them on acrylic nails. You shouldnt use it on your natural nail. But there is a buffer so that you can buff your natural nail to make them shinny!
They use them to even out the acrylic nails when you get fake nails on - i dont think its wise or safe to use it to file your real nails - buffer it just like a soft thing that goes across to make them nice looking - like a car buffer at the car wash.