Why do they change pitchers to pitch to a right hand or left hand batter?

right handed pitchers throw breaking balls that break AWAY from right handed hitters and vice versa, thus it is generally considered more difficult for a right handed batter to hit righties. There are exceptions, the 'screwball' is an example. Generally a right hander wants to throw it only to a lefthander. Its not that common of a pitch. Recently the most notable example was Fernando Valenzuela and lesser known Jim Mecir.