In rural areas where this is common there isn't traffic. Thus the politeness doesn't cost you anything. Also in those rural areas it is somewhat common for someone with the right away to have a reason to drive much slower than the speed limit and so giving up your right away makes things better for everyone - the other person doesn't have to try to pass you latter.

When there is traffic you giving up the right away is impolite to the person behind you and shouldn't be done. unless traffic is so heavy that you won't be able to move, and so you should give you your right away to someone who can go.