What is the point of a 20 MOA rail?
For long range shooting, you have to aim above the target in order to hit it. For example, a 308 will have dropped 340 inches at 1,000 yards.
Instead of aiming above, it is possible to dial a scope in so that you can hold the crosshairs directly on the target. However, scopes have a limited amount of movement. A 20 MOA (or 30 or 40) rail will point the rifle up 20 MOA so that you need less scope adjustment.
At 1,000 yards with a 308, you would need to adjust up 34 MOA, while with a 20 MOA scope base, you would just need 14 MOA of upward adjustment.
This is more common on long-range rifles because with most rifles you don't need to be aiming 20 inches up at 100 yards, and so you would have to aim low at shorter distances.