When shooting an arrow at a target why is it advisable to aim above the bull's -eye rather than directly at it?

The arrow does not travel straight, it travels in a parabolic curve from the bow string to the target. The action of gravity pulls the travelling arrow down, so that by the time it gets to the target it is lower than what you initially aimed for. If you aimed for the bullseye, then your arrow may hit lower than the bullseye. Aim above the bulls eye--but not too high--you will hit the bullseye.