It's one of those laws that many people know but don't really think about when driving. However I guarantee you everyone has experienced this scenario (I do on a daily basis taking Hwy 31).

An article from Texas Highway Man sets the scene:

You're in the left lane, going faster than the flow of traffic until you pull up behind someone going slower than you. Naturally, you go around by taking the right lane and then you return to the left lane as you speed on by.

This happens all of the time and it seems harmless but you are actually creating more hazards than necessary when driving. Sometimes there will be someone getting on to the highway (at slower speeds) while you're passing someone using the right lane.

You're also most likely slowing the flow of traffic because instead of the slower car moving over to the right, there's now a line of faster drivers preventing that slow car in the left lane from moving over. Now you have mixed speeds in both lanes and the flow of traffic is disrupted. Thanks a lot!

Give the slower driver on the left a few seconds to realize they need to move over to the right. They usually do.

According to Texas Department of Transportation (TxDOT), "impeding the flow of traffic by continuing to drive in the left lane is punishable by a fine of up to $200". So if you're not actively passing somebody, you should be in the right lane.

So in all, stay in the right lane unless you're passing a slower vehicle. If you're in the left lane and you see cars lining up behind you, move over to the right lane so everyone can follow the law accordingly.

Essentially, it's teamwork where everyone knows their role on the team.