Every state has some kind of law restricting the use of the left lane on multi-lane roads and highways. That doesn’t mean you aren’t allowed to use the left lane at all — it just means that you should generally use it only to pass cars in the right lane.
Why is that the case? Even if you’re driving fast, there’s almost always someone going faster than you. So if you get back over to the right immediately after passing, that car will be able to pass you, which lets everybody to get to their destination more quickly. Otherwise, traffic builds up, raising safety risks.
The autobahn is a living testament to what our road could look like if everyone followed this rule. The German highway system boasts lower accident and fatality rates even though it has higher (and sometimes nonexistent) speed limits. It isn’t just a matter of courtesy to the people driving behind you — it’s a real question of safety.