A fin provides directional stability at high speed. Given that the straights at Le Mans are surrounded by trees etc, you probably have less cross-wind issues, so when you want to achieve low drag at a circuit like Le Mans when the Mulsanne and the run to Indianapolis make up such a high proportion of the lap, without the feeling of everything going light and wobbly at high speed, perhaps it means you could run less aerodynamic downforce, so less drag, and a higher speed, or the same speed for less power and less fuel consumption on the straights??
MG Mark