I feel the term "scope leveling" is misleading ,in that it is more scope alignment to the bore, and setting the cross hairs( reticle ) plumb vertically. scope levels come in to play to help with this. EXAMPLE I put my gun on a stand , level the stand / gun , mount the scope to the desired eye relief . now time for the "scope levels" put one level on the gun breach for "cant "adjustment and the other level on the scope to align the two .I then use a string like a plumb bob to confirm that it is good. bright fishing line with a weight also works good. The only real leveling is to the stand/gun the rest is cant alignment and crosshair plumb. THIS IS MY METHOD not gospel or maybe even correct ! I am not a scope guru or professional shooter, nor am I saying others are wrong. but I have been using this method for quite some time and I find it to be easy and effective for ME. Scopes are a personal preference deal for each person, size, magnification, eye relief and reticle choice . A friend of mine prefers a slight cant to his scope as to how he holds the gun, so if I were to look threw the scope with my hold of the gun, it looks out of plumb.. Again personal preference , but I would not use his setup or he mine.
This post isn't about scope leveling though, its about scope levels. As in the tool to avoid cant when shooting. Perfectly aligned scope should be the case regardless, but then if the shooter holds the gun at a slight cant (sometimes not even noticeable through the scope) you could be missing by 2 to as much as 12"+ at 100-200yds just because the gun is slightly tilted. Especially if you are not shooting on a leveled bench.
Like I already said at 50yds and under it doesnt make a difference, but next time you are shooting to 100 on a nice solid rest.. just do a little experiment. Shoot 5 shots then slightly tilt the scope enough that you know you tilted it but that you really cant see much difference (if any) in crosshairs... I bet the next 5 are in a totally different place.
You will rarely see them on competition shooting because they are either shooting bigger targets under 50yds where it doesn't matter, or they are shooting from a bench and have already used a pocket level to level the gun on bipods and locked everything down.