Other than when the lack of sightlines demand it, mostly it's to deal with the situation where there's a tiny, broken, or non-existent shoulder on a narrow road and you end up with people passing within inches of you. Sometimes that's because there's an oncoming car, but most often drivers appear to believe that the Hand of God will smack them dead if they even contemplate moving over the center line to pass a cyclist, even with nobody coming the opposite way. Moving out into the lane seems to trigger a reflex where the drivers eventually realize "Hey, I actually CAN move into the opposite lane to pass this person safely."

Edit: Just as an example, here's a vid of my 13-year-old and I riding on a typical local road this past spring. Controlling the lane would probably have stopped the car coming up behind us from doing his best to have a head-on collision.

https://www.youtube.com/watch?v=qDXQbCPRlHY