What is the purpose of "D" Rank?

This question comes to my mind a lot even though we already have C ranking and it itself is a very bad rank.

I already swallow a lot of strange philosophies behind this game that I do not want to discuss now, but having S+/S/A/B/C is more than enough.

As a final note, we as a community aim to improve the game, so let's discuss it civilly. You never know maybe what you write may be new future in the upcoming updates thanks to YOU.