If you take a close look at the adjacent diagram, you will understand what a roller box arrangement looks like. The four parts shown in the sketch consist of two rollers, the work-piece and the cutting tool which are not labeled on the diagram itself but can be easily singled out.
You can easily imagine that when the cutting tool is performing its operation, the rollers perform the function of maintaining concentricity on the work as well as to prevent deflection of the work piece and the end result is a much smoother finish, than what could be obtained using the cutting tool alone.