A pitcher can be either left- or right handed, but the better ones pitch reasonably well to both left and right handed batters. When you get to the fifth starter on the roster, however, many teams are dealing with marginal throwers.
Some bating teams have relatively more of left-or right-handed batters. So use one pitcher against the one set of teams and the other pitcher against the other set of teams. Jeff Locke and Brandon Cumpton of Pittsburgh come to mind. One is a lefty and one is a righty.
Are there any teams that in fact, do this, because they have "fifth" starters on the roster who throw a lot better against either left- or right handed batters? Perhaps combined with another back end starter who has "platoon" splits the other way? Meaning that it would make sense to "platoon" a "backend" starter role so that the tandem may actually be better than a fifth starter?