It breaks a siphon. ON a hot day, with the tank really full, the fuel can back clear up into the fill tube, especially parked on a hill AKA Seattle/ San Fran, and siphon out the vent onto the ground. With the "U" as soon as the fuel gets low enough it will pull air and stop. The fuel cannot flow out again, until it backs clear up to the top of the U level wise. If you are driving, you may use enough that it won't siphon again, or if cooling towards the end of day, the fuel may subside on it's own.
If you can silver braze or conventional braze, or even gas weld, you could braze a fitting into the neck On a car where the neck goes in from the outside, you need to keep the fitting fairly flush so it will clear the fender hole