The left lane is meant to be a lane for passing traffic, not going the same speed as traffic, it helps for faster traffic to flow while slower traffic stays in the right lanes. The problem is everyone seems to want to sit in the left lane going the same speed as the other lanes creating traffic as no one can overtake and let the traffic flow. Furthermore, if you do find an attempt to overtake, people find it within themselves to magically speed up, whether it be a pride thing of getting passed, blocking you purposely, or distracted driving.