This one observation can give you a tremendous amount of information about someone. Information that can come in handy, on down the road. You can use it to better predict their future actions, which in turn can be used to better time you accelerations and decelerations.
Side note: Providing room/‘making a move’ for others isn’t only for their benefit, it’s also a valuable communication technique for anyone anyone who observes it. Indeed, many thousands of accidents could have been avoided if someone would have made a seemingly unnecessary move JUST to raise awareness!