For argument sake, lets say that your average rifle with a flat scope base and equal height rings will mount your centered scope so that you are zeroed (within a couple clicks) at 100 yards. If you have +/- 40 MOA of adjustment (+/- 160 clicks, usually), then you have about 40 inches of rise or drop worth of adjustment, at 100 yards, to play with for use with high or low powered rounds, or for stretching out downrange. You're not likely to use much of the up, but it's possible to run out of down, requiring you to hold over.
Throw on a 20 MOA base and now you're pointing 20 inches low at 100 yards. So you crank your scope up 80 clicks to get your zero, and now you have 60 MOA (240 clicks) of down adjustment available for those long range shots before you have to hold over. That's another 8 feet of drop at 500 yards that you don't have to guess your hold on. Over 16 feet at 1000 yards! And still no problems up close.