Airlock Question

This is probably a stupid question, but I will ask anyway.
In everything I have read they always say to be sure airlock is filled 1/2 way with water. What is the purpose of 1/2 way? What happens if it is more or less than 1/2 way.

If is under filled it will not bubble and the risk is there that if there isn't enough water then floaties can infect a brew (I guess) and if there is too much water in there than the risk is that some of the water can end up in a brew causing an infection. I'm guessing as I know of nothing that will infect a brew after a sanitized airlock has been inserted weather water was in it or not, assuming you mean a 3 peice with cover.

...close enough...

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.

I use Star San left over from sanitizing brewing equipment in my airlock, as I'm sure other people here do also cause I'm sure this is where I got the idea. I have it around... unlike vodka which I can't think of any good reason to purchase, let alone have it come anywhere near my beer. Who knows whats in that stuff!?! Although I suppose vodka doesn't foam nearly as bad as Star San.

Being the curious person I am I once drank a bit of the vodka from the airlock after like 3 weeks. Carbonated vodka is disgusting, never do this.