Per Boaz Aviram: I basically taught it without even knowing that it is used by some boxers at the time. I just memorized the natural motion I used to reach and knock out a karateka in 1980 after learning the boxing/Krav Maga punching style, and was teaching it ever since!
Realizing that efficient fast non projected reach to the target is one of the essence in hand-to-hand combat, it was just natural! The drop punch seems to be the most direct non-projected entry with a punch into the hotzone!
From scientific perspective: The human reaction time is between 0.2 - 0.25 Seconds The Drop punch demonstrated in the video Pure Krav Maga - Self Defense Mastery was 4 picture frames at at 30 frames per second = 0.1333 second. So from non projecting neutral position one can drop a punch from out of the hotzone into the hotzone and the opponent's face before he is able to complete his reaction time(time from identifying the danger, processing it and sending a command for the limbs to start moving measured by initialization of the motion.
So even if you are a little less efficient in your non projecting motion, it would be hard to respond to your drop punch timely. Also when you are doing the drop punch your body is hovering in the air with forward motion, so you have 100 percent of your body mass to support the strike...Get the Training Manual: