Guides

The elevation adjustments for your scope reticle have a finite amount of vertical range.  The Monstrum Challenger, for instance, has 45 MOA of total adjustment range.  

If you zero your scope in the center of the adjustment range, then the scope has only 22.5 MOA of downward adjustment range.

So the amount of bullet drop you can accommodate for is limited, the adjustment range will max out for long range shots (the distance varies depending on the caliber). For instance, 55 grain 223 has a bullet drop of 28 inches at 400 yards - at distances of 400+ yards, you will be unable to adjust the reticle to compensate for the bullet drop.

A 20 MOA scope mount angles the plane of your scope downward 20 MOA.  It thus shifts the reticle adjustment range downward by 20 MOA.  The reticle is zeroed at a higher point within the adjustment range of the scope,  allowing you to position the reticle for longer distance shots with more bullet drop.