Why do i need periodic boundary conditions in order to calculate reflection. c?

Why do i need periodic boundary conditions in order to calculate reflection. coefficient? I dont have a periodic or infinite structure, i want to calculate it for cube. But when i run feko solver, it requests me to set some periodic boundary conditions. I dont get it.

I use a plane wave as a sourse. Should the plane position for phase reference be by (0,0,0)? (because it should be orthogonal to k propagation direction)