It assumes drivers are not going to be assholes. But that is a big assumption.

But the idea is that it neutralizes speed disparities between lanes and allows for an organized pattern that people can anticipate. This makes it both faster and safer.

If you have two lines of people approaching a choke point, you just all agree to alternate where one person from one line goes through, then the person from the other line, and back and forth.

Without that, it's a mess because five people in a row from one line might try to shove into the space, or two people from two lines go at the same time, and then it rapidly devolves into every person for themselves because you can't anticipate what others will so.

Merging at one point rather than any random point causes less brake checking

also if you are trying to merge on to the highway, the on ramp lane is that long so that you have enough time to pick up speed to keep up with traffic before you merge, rather than merging at a slow ass speed, causing the people behind you to brake suddenly.