It doesn't do anything when the camera is behind the car (close view or far view, as it is also known).
However, the cameras in the cockpit, on the hood, and on the bumper are affected. If you set some keys that control the character's head, you'll find that when you press the keys, the head turns much faster at 5x than when the setting is at 1x.
For example, I set my look left control to the delete key. I also set my look right control to the page down key (you don't have to set it like mine). If I want to look left at 1x, the turn is very smooth and quite slow. At 5x, the turn is fast and sort of like a jerking motion.
I believe the 5x function is commonly used for racing, and the 1x is commonly used for slow driving. Also, at 1x, the head turns at a full 90 degree angle. At 5x, it's a bit lower than that.
Sorry for the long response, but I cbb to make a video to show you.