What is the origin or reason there is a screen attached to the foul pole?

Answer 1

It makes it easier to call a home run or a foul ball. The call is made by standing on the baseline and looking into the outfield, but the ball is small and mistakes can be made. By putting the screen up the ball that doesn't hit the screen is usually so far fair that its easy to make the call. Since home runs generally 'hook' around the pole, the screen extends from the foul pole into fair ground because if it hooks foul, it also tends to be fairly obvious. If the ball HITS the foul pole its fair and that also tends to be obvious too, usually the deflection is immediately noticed. Mistakes still get made though. I have seen a play where a player hit a short home run over the left field wall which bounced back out, and was fielded by the left fielder. The ball was definitely gone on the replay, but the umpire somehow had a poor view of the play. The way the ball bounced looked more like a shot off the outfield wall than a home run.

Answer 2

The grates are there to aid the umpires in determining whether a high fly ball entering the stands is fair or foul. The grates extend toward the fair side of the pole. A ball passing close to the pole that hits the grate is obviously fair. If the grates were not there, it would be more difficult to determine whether the ball is foul or fair because of the ball's close proximity to a relatively "skinny" pole. It is assumed that the umpire would be able to tell that a ball is fair if it passes cleanly to the fair side of the pole and doesn't hit a grate that is probably a foot wide.