What is the purpose of this function in shift+u / free camera?

It toggles between "Following" and "Locked on", I notice no difference.
How to use it?

Vague memory but I think if you lock it then it stays locked to that car even if that car is not visible the way you are looking.

When you are following a car, it can be quite easy to un-follow it accidentally while moving the camera around. The lock keeps the camera locked to the selected car.

thanks, hm yes seems to be something about 'how easy' the camera loses tracking.