From the manufacturer:
"Use of the Arisaka Optic Leveler makes the assumption that the scope's turret cluster and reticle are aligned, which may not be the case with poor quality scopes."
It also makes the assumption that your barrel is perfectly straight, the bore-line is perfectly straight and centered in the barrel, the dovetail is perfectly centered over the barrel, and the scope mounts are perfectly centered on the dovetail.
That device lines up the bottom of the scope with the top of the receiver/dovetail. For "leveling" the scope to your test bed, it's fine, because you have no bore-line to worry about.
On a lot of my guns, if I relied on that device, the bore-line/reticle vector would end up slightly canted.