“Because the light is reduced, the shooter’s eye acts like a camera lens and adjusts the depth of field (range of distances at which objects appear in focus) to the maximum. That’s what keeps both the sight picture and the bullseye in sharp focus, but the shooter wants the front sight to be in the sharpest focus, because it’s what he focuses on.”
So you get a sharper target while focusing on the very close (by comparison) sight.