It's done for the same reason you do thousands of reps to build 'muscle memory' for fundamental shooting skills. The more you run through (and visualize the stage run), the less you have to think about what you're doing while your doing it. Consciously thinking about what you're doing slows you down. If you can let your subconscious take over, you can shoot much faster. On my best stages, I've felt like I was just an observer along for the ride.
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