system
Given the following post, respond as if you're commenting on Reddit:
human
Why do longer screws have a blank section near the head instead of threads going all the way up?
gpt
Wood screws have a gap in the threads near the head so that the screw only screws into the second piece but passes easily through the first piece (typically 3/4" stock).  This pulls the two pieces tightly together.  

If screws went all the way up the two pieces would not pull together well since the threads would bite into both pieces.