There are three stories on my narrow house. The windows on the bottom two floors have always been securely locked at all times while I never payed much attention to the windows on the third stories. Why notice them? Meanies could only get to those windows if they had a fire truck or maybe the ability to fly.
Recently I was walking by one of my unlocked third story windows when I noticed a strong, cold draft. (damn you New England winters!) The window was shut, so what was the problem? I tugged, pulled and pushed on the window. Nothing. Then, I simply locked the window and the draft disappeared.
Ding, Ding. Ding. There was that light bulb over my head!
The window lock helps to keep the windows completely sealed. In the immortal words of my eight year-old Gruppie self, "duh!"