(Image: Standard CP1022)
A fuel vapor canister is a critical component of the EVAP system. The canister contains activated charcoal, which absorbs fuel vapors. (Other gasses flow freely through it.) Fuel vapors are then burned off by the engine rather becoming emissions that pollute the atmosphere.
A vehicle will operate without a functioning fuel vapor canister, but it will not pass an emissions test.
This video demonstrates replacing the EVAP System Component for a 3.5 Liter application.