The purpose of the airlock as I understand it is to allow CO2 to leave the fermentor. You don't have to use water, many use Vodka. The water or whatever you use acts as a barrier... if you didn't have this barrier that the CO2 could leave with and just put a stopper on it, well that wouldn't be good right. The pressure would just build up till the bottle exploded. On the flip side if you did not have the barrier at all then like boo boo said, you run the big risk of infection. I imagine before this was thought of much went bad real fast.