rear tank - fill neck venting

[cjohnson6772]

I have installed a FuelCellGuy rear fuel cell with my swap. There is a vent line that comes out of the passenger front of the tank by the fuel pump. I just hooked a rubber hose to it and have it vented to atmosphere. There is a 3/8NPT bung that is labeled filler vent. Typically we see these ran by to within an inch of the cap. Is this necessary and why? Can it not just be vented to atmosphere towards the back of the tank just like the other side? I really don't understand the difference...
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