I am 99% sure that is called the "EVAP can". I remember some argument with the emissions guy about it not being there after the SC install.
On the emissions diagram under the hood it is shown, and you will fail the visual emissions check without it. But as for what it does... I don't have a clue
2001 Ironman Daily Driver... 3.5" suspension lift (OME912 springs and 1" spring spacer), ball joint flip, 1.5" front diff. drop, 33"x12.5 TrXus MT, 16x10 Eagle Alloy rims, Interceptor, PV muffler, K&N air filter, Alpine Supercharger, Bilstein shocks, and some trimming.
2000 Ironman Project LS-1 VX... very slow progress but someday....... ohhhhh someday......