Most of the time when a pool cue is “crooked”, it’s not because the shaft or the butt are bent, but because the mating surfaces on the butt and the shaft are not perpendicular to the parallel axis of the cue. By facing the surface on the shaft at certain locations we can straighten out a crooked cue, with the shaft/butt already straight.
Reminder: this only works if your butt piece and shaft are straight to begin with. We are addressing a concentricity issue only at their mating surface. Bent shaft is easy to fix just by looking and bending in the opposite direction of the bend. Once you have two straight parts, then do this.