Mini filler neck and gas cap

[RICHARDSMINI]

So, for some reason my mini 04 dies but can be re-started by simply taking off the gas cap for a second. But, my question is, at the top of the filler neck there is a hose. What is the purpose of that hose? I've seen diagrams where some cars seem to have several hoses coming off the neck near the tank. Cheers-RKL