Most people ferment with just a cheese cloth over their primary buckets for the first few days then cover them up with lid and airlock or rack to carboy when SG hits around 1.020. When you start a batch of wine and add the yeast and it starts fermenting this is an aerobic stage meaning it needs air for the yeast to grow and populate, what it is doing is acyually inceasing its popultion by around 10,000 times the amount that came in the packet, once its done fermenting it has reached its anaerobic stage meaning that 02 is actually harmful yo your wine. The part about getting it uder airlock before it actually finishes fermenting is that you want some of that C02 that your wine produces during fermentation to protect your wine when it does finish fermenting . C02 is heavier then air and will give your wine a blanket of protection.