Dosing vodka, sugar and/or vinegar (carbon dosing) increases the bacterial activity/mass, which resulted in reduction of nutrient. If i remember reading somewhere that a protein skimmer is necessary to increase oxygen level and also to skim out bacteria which has done their "job", else the nutrient will be released back into the tank.
WC does not only "dilute" NO3, PO4, it also replenishes trace elements for your reef. Hence, WC is important.