Sometimes you want different objects to have opposing screen directions. In those bad old western movies, the cowboys rode left-to-right while the Indians rode right-to-left. That created the impression that they were in opposition—and would eventually engage in battle.

But what if the Indians were going right-to-left and the cowboys were also going right-to-left. Then what would the audience perceive? Viewers would think they are working together!

To the right, is an example of how this works.  In this old WWII movie, notice how the Americans work left-to-right, while the Germans work right-to-left.