Crow Country is filled with secrets and unlockables, many of which are a lot easier to reach on a second playthrough. Achieving an S-Rank allows you to unlock a tremendously powerful item for future playthroughs, though by the time you've successfully got an S-Rank you'll probably have tracked down most of the game's secrets. Getting an S-Rank is also necessary to get all of the game's achievements, making it an important step for completionists.