Is the fuel return shut-off valve really necessary?

Installing the shut-off valve on the fuel return line allows the fuel level in the carburetor tank to be adjusted with precision. This solution allows you to correctly calibrate carburetion during a race, in particular, if you feel the engine too "high-pitched" on the straight, you can make the mixture slightly richer. Naturally, in an ideal situation, in which carburetion has been optimised before the start and in which nothing particular happens out on the track, all this should not be necessary. You could say that the shut-off valve is an extra safety guarantee that increases reliability.