If I am within 30' & behind a car (looking to pass) I ride in the left tire track of the car. My theory here is, if i'm in the center of the lane, and there's an obstacle in the center of the lane, odds are the car ahead of me won't move left or right to avoid it. I wouldn't have any idea it it was there until the car ahead of me passes over it, and I would have little time to react in order to avoid it.
On the other hand, being in the left tire track of the car, if the obstacle is in this area of the lane, the car will move left or right to avoid it. When I see a car moving, I move along with them, anticipating that there is an obstacle I can't see in my area of the lane.