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.
The other use case for this is long range shooting with 22lr, the drop is between 18-30 MOA at 200 yards due to the low velocity of the cartridge, especially with subsonic rounds.