purge

Hi all, Just a quick question.... Can someone explain the purpose of the small rubber hose that connects to the airbox, just under the maf inlet? I've heard it referred to as a purge pipe, but what exactly is its function? thanks!!

I had a strange ticking noise in my engine bay, it only happened at idle. I took it to the garage today and they said that its supposed to be there, its a purge valve or something. the box where its coming form is next to the windscreen washer bottle. does anyone know what this purge thing...