The reason the follow-through is so important is it ensures the full capacity of your quad muscles fire during the kick.
One of the main things Backstrokers have to deal with that Freestylers never really need to think about is–the water line. Obviously in Freestyle, it’s super important to have great body position. You want good alignment and keeping the body up at the water-level with good alignment is pretty much the entire goal of swimming (if I were to dumb it down for ya)–but we all know it’s not that easy or super possible.
For Backstrokers though, keeping good body position is a bit more tricky because they are flipped over to their back. This allows for more mistakes to be known—like the knees breaking through the surface during the Backstroke kick. You would never see that in Freestyle, but in Backstroke—it’s super obvious.