bull...it purges a carbon canister while the engine runs, so that it doesn't vent the vapours in the atmosphere
I guess you could live without it in a custom application.
Same as the hot-start valve (on the throttle body)
If you 'T' it back into the fuel preasure vacume hose...i.e. creating a loop ...you can leave the vent valve plugged in and not have any engine management light problems.
I've been running this on my LET in my astra since i did the conversion with no probs.