Auto fill

So can somebody remind me again why this dog shit feature is in the game?

Just played a game. I queued for mid over support. I'm auto filled jungle. My mid laner is autofilled mid.

So can somebody explain to me why we had two autofills when instead we could have had one autofill? The game was a complete waste of everyone's time and LP.

I really want to understand because it appears this system is designed by an elementary school student.