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!"