I think it's just simple politeness and manners - if someone needs or wants to go faster than me and I am in the left lane, I will move over for them unless I am also actively overtaking cars on my right.

I see it the way I see aisles at a supermarket - sure I have the "right" to block an aisle with me and my cart, but if I see someone wanting to get by, I'll move over to the side for them.