Bit of a stranger to this place these days but hi to those that might recognise or remember me

My question here is in relation to fuel tank pressure. Is there any reason or science behind having pressure in the fuel tank on injection fuel systems? Something ive been wondering in a while and have been meaning to ask.

I realise that old cars such as ERST's the tanks are vented to atmosphere, but that all changed with emission laws and such so that all vapours were re-circulated into the engines intake.

This doesnt relate to a ford, but its a fuel injected car with an in-tank pump, so does anyone have, or know of any reason why i cant let the tank have a vent to atmosphere?

Thanks in advance people