I'll make this one short and sweet, because it really doesn't need any elaboration. The left lane has another title that it likes to go by - it is the PASSING lane. Which means, that it is designated for passing. Definition of passing? - Moving at a speed which allows you to go past the car(s) in the other, non-passing lane(s).
So, just to recap: If your not passing and/or are unable or not capable of passing, move your car safely and quickly out of the left lane (also known as the passing lane.)
*This is such a pet peeve that some states, such as Illinois and Kansas, have a law against left laning it, which means that you can be ticketed for being a jerk.