Just allows you to time stuff without having to click the reset button. But it doesn't allow you to time the difference between two things. In the modern day it's not that useful, just a cool feature that showcases watch making.
But back in the day, imagine you were a pilot/navigator relying on exact timing. You go from point A to B for 10 minutes, then B to C for 15 minutes, C to D for 7 minutes, etc, the flyback allows you instanteously reset timing without wasting 10 seconds fiddling with start reset start (especially with thick gloves).
The timing the difference between two thing is a split second chronograph or rattrapante, which is much more complex.