The same the check valve does on your brake booster (and the cruise control on older cars)... Your engine makes the most vacuum at idle, so at idle, it makes an almost perfect vacuum in the vacuum canister, so that there is more "power" to actuate the throttle when you're at cruise...
BTW, ever have a check valve go out on a brake booster? It's REALLY cool! when you go directly from the gas to the brake, you have basically manual brakes until the engine goes back down to idle, then you get full brakes again... really neat huh? This is really only an issue with automatics, BTW... you have a VERY high vacuum when you just close the throttle while you're in gear on a manual...
Mike
umm.. What would change if I didn't use the check valve then?