When you seal the polyethylene sheet at penetrations and laps, you maintain the continuity of both the air and vapour barriers simultaneously.

When you apply caulk around an exterior window frame, you are sealing the air barrier at that location — but the vapour barrier function is handled by the interior polyethylene sealed to the window frame.

The practical implication is that air sealing must address both the interior and exterior faces of the building envelope to be complete — sealing the interior air barrier stops warm, moist air from exfiltrating into the wall cavity, while sealing the exterior air barrier stops cold outdoor air from infiltrating.