Platoon Advantage
A hitter sees the ball better from a pitcher throwing from the other side.
How it actually works. A breaking ball from a same-handed pitcher moves away from the hitter and starts behind him, which is the single hardest look in baseball. Opposite-handed, the same pitch starts in the hitter's eyeline and breaks toward the plate.
Why it is done, and when it is wrong. This is what half the moves in a late inning are about, and it is why a left-handed reliever with modest overall numbers can have a long career. The effect is far larger for left-handed hitters against left-handed pitchers than the other way round.
Batter's Eye: look up anything in baseball