Yup! It's for emissions. Federal reg's require that little or no fuel vaoprs escape to the atmosphere, thus all fuel vapors are routed to the vapor storage cannister. The cannister contains charcoal granules which are contained in a nylon like stocking. The granules hold the vapors until the proper time when the vapor purge solenoid is activated and allows the vapors to be routed to the intake system for combustion.
Mike