Fuel pushing out filler tube

The only fuel going back to the tank first came from the tank, basically a loop. No pressure build up even if the tank had no vents. you are consuming some of the fuel in the carb/efi so you have a net loss. This could create a vacuum in the tank if there were no vents

Example...

(Just numbers, not real world values)
You have 5 gph being pumped out of the tank, 1gph is used by the EFI / carb, the return returns 4gph back to the tank. With no venting the tank will develope a vacuum and either crush the tank or stop moving fuel. With a vent the pressure in the tank will be atmospheric or very close to it but it will never be pressurized.

Even with the pump running and the engine not running you are still pushing 5gph out and returning 5gph, no vent, tank has no pressure or vacuum in it, with a vent tank has no pressure or vacuum in it.