Maybe this diagram will help explain the effect of rifle cant and how to calculate the effect. The drawing isn't ideal because no one would cant their rifle as far as even the first angle shown, but I think it can explain why it's important even for small angles.
In this diagram red dot marked 1 is both the point of aim of the sights on the rifle and the target (point of bullet impact without cant) . The red dot 2 represents the point above the target where the bore is pointed. The vertical linear distance between dot 2 and dot 1 is the total drop of the bullet at the target, in inches for this example,, not in MOA or Mils. The green dot 3 is where the bore will be pointed (in the plane of the target) if the rifle is canted (rotated about the target (dot 1) . In this picture the angle is 22.5 degrees (1/4 of 90) which is rediculously large, but it will do for example. Green dot 4 is where the bullet will impact in the plane of the target with the rifle canted.
Assume the target is 1000 yards, the rifle is a 300 Win Mag, the bullet is a 190 Sierra Match King. I'll use 5 degrees for the cant angle. That would be hard to sense in windy mountain terrain with no external physical clues as to where vertical is. A ballistics program shows that the total vertical drop at the target Dot 2 to dot 1 on the drawing from the combined effect of gravity and atmospheric drag will be 327 inches (2950 fps mv @ sea level)
The sine of 5 degrees is 0.087 times 327 = 28.5 inches. for the horizontal error. The distance horizontally from dot 1 to dot 4 at the target.
The vertical error will be (1-cos( 5)) * 327 = 1.24 inches. The distance vertically from dot 1 to dot 4 at the target.
For practical ranges and small can't angles you can ignore the vertical component of cant, but the horizontal component is quite significant and cannot be ignored for long range shooting. Even a 1 degree cant error will be a 5 to 6 inch error at 1000 yards for typical hunting rifles. Actually cant error can be accurately calculated only from bullet time of flight and the cant angle without any other information about the rifle or bullet since drop in inches is always 192xT^2 where T is the time of flight in seconds.
Why can long range benchrest shooters get away without considering cant? Because they are shooting groups and use sighters to null out the effect of cant and the benchrests insure that the cant doesn't change between shots. Those are both luxuries hunters usually don't have. Most do level the benchrests and square up the rifles and the rests even if the rifle it'self doesn't have a level affixed.
The sine and 1-cosine calculation only apply to small angles and aren't exact. The sight displacement from the bore isn't in the calculation nor is uphill/downhill shooting considered. . The purpose of this exercise isn't to calculate the effect of cant int the field. It's to show why having a level on the rifle is worthwhile for LongRangeHunting.