Fuel vent line

hey guys im intalling fuel line on my 71 demon and i was wondering if i need to run the vent line back up front or if i can leave it disconnected, also what purpose does it serve? emissions?

I think if you leave it disconnected you will be pouring fuel out of it
as soon as the fuel level covers it up. I plugged mine on the 'Cuda
as I have a vent on the filler tube.