The session is hours long; the window inside it where the move actually starts is not. The kill zone is that window, and it is the difference between a setup that means something and the same shape at the wrong time.
Why narrow the window — Most of a session does nothing. The kill zone is the part where the sweep and the expansion usually begin, so it is the part worth being at the desk for.
It starts moves, not finishes them — The objective is often delivered later, outside the window. The kill zone is where the move is set up, not where it ends.
Time is context — The same setup inside and outside the window are different trades. That is the whole reason to mark the clock at all.