the true reason to do a full S rank run of a zero game is because you find them >!fun!< and want to try getting the most out of the game
outside of an incredibly messy picrel for the final stage, i loved my S rank run of Z1. i had one going for Z3 as well, but stopped at volteel biblio's stage because it was pretty aids to play the stage. i might get back to it someday tho