It just makes the PID slider for each move in sync with the other, so you get the same PID for roll as you get for pitch. This is often used on quadcopters where front to back is the same as left to right.
Be aware that inertia and PID isn't exactly the same when the length of the battery is say fore and aft. There is slightly more rotational inertia fore to aft than left to right in this case. The difference isn't usually worth worrying about for a typical (non-professional) user.