Picture artillery in the field. You're looking straight out across the battlefield through your sights, but the barrel on that cannon needs to tilt way up to lob that long-range shot.
Here's another of my incredible, award-winning illustrations.
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^^^ This right here.
I mounted an EGW 20 MOA scope base on my 10/22. The thick part is to the rear. Mounting it this way (the only was possible when you look at the thing) gives you 20 MOA of additional headroom to dial you scope in for long-range shots.
My 10/22 reaches out to 200 yards and, IIRC, I dial in about 43 MOA to do it. If the scope was mounted on a flat rail, there wouldn't be enough adjustment in the scope to dial me in at 200 yards.