A fastball covers the distance from the mound to the plate in less than half a second, requiring lightning reflexes and judgment from the batter to even make contact. So when the hitter does make contact, he has to try to make the most of it.
As Brian Johnson, formerly a catcher with the San Francisco Giants, describes it, a hitter has to be aware of wind and atmospheric conditions to get the most out of a hit: "Sometimes the wind might be blowing somewhat in towards the plate in left field, and slightly out of the park in right. In that case, you might take advantage by trying to hit the ball towards right."
In order to take advantage of a wind, hitters may try to alter the trajectory of their hits. Former Oakland A's star Rickey Henderson says: "When the wind is blowing in, you really try to keep the ball out of the air as much as you can, and try to hit the ball on a line drive that can get through the wind.