The technical reason(s) are these, listen closely, when you have a high horsepower car (300+) the last injectors on the rail get starved of fuel without a fuel return line back into the tank. The return allows the fuel to evenly go THROUGH the rail offering all the injectors the same amount of fuel. Also, a return system uses a 1to1 fuel pressure regulator, per every psi of boost you'll get a psi of fuel pressure! Hope this clears things up
