Driving to work is never a fuss-free affair. Even at 7.30 in the morning, the expressway that I use is packed with cars. The problem with traffic flow is people do not drive at uniform speeds. Suppose there is a 10 metre gap between you and the car in front of you. You speed up to close the gap but by the time you close it to 5 metres, you have to slow down. It would have been better overall for you to keep at your speed and save the unnecessary acceleration and braking. Moreover it creates gaps, and irrational drivers like to cut into lanes whenever they see a gap. Most of the time they do it abruptly and following cars have to at least slow down to allow the rogue vehicle to change lane. This is akin to turbulence in a fluid flow. To that person who changed lane, he might have felt that he advanced* but overall traffic flow is actually reduced.
Slightly related to the above problem is what is known as Braess’s Paradox, which in a nutshell says that adding extra capacity to networks might not improve flow if people are irrational. So even if an additional lane was added to the expressway, it might not improve flow as much doing something like forbidding the changing of lanes. Wiki has a very clear explanation of the paradox with a very enlightening example.
* That is not even taking into account the psychological perception that whichever line you are in, it’s always the slowest. So irrational drivers would switch lanes whenever possible.