Not exactly. It's function is to contain fuel vapors on a constant basis. Cali bikes vent the tank through the carb rather than the fuel cap, via the canister. Ain't no thing, ZERO effect on performance, just another way of venting the tank. Folks are probably tired of hearing me say it, but I've become a big fan of them. No fumes in my garage full of bikes, and if you've ever had the necessity of dragging your bike into a Mexican hotel room for the night you'll appreciate the extra brain cells in the morning.
I used to take them off, day 1. In Cali it's practically a ceremony we all perform without knowing why. "Decalifornication". "Stick it to The Man". Unless they weigh more than 3 or 4 pounds I leave them on. The TW's weighs 1 lb., even.