What is the purpose of the batter board?

Answer 1

I assume "batter board" is another term for "Batters Eye" in center field. If not then this answer will not apply to your question.

The Batters Eye is for the batter to have a "backdrop" that will allow him to see the Baseball come from the pitchers hand. Generally these are green or black because they allow the white ball to be seen easily. If you are in the outfield on a line drive hit to you, then you might appreciate having this as when you are at bat. A white ball being thrown at you 80-90mph that you cant see because someone in the center field bleachers is wearing a white shirt right where the release point is for the pitcher would not be a good thing.