The Fairy Godmother is one of the Wish Granters, a team of fairies sent to make dreams (good ones) come true. She is usually assigned specifically to whoever the Devils have their eye on at the time. Since they were hoping to break Cinderella through their agent, she was assigned to watch her, but only to step in when absolutely necessary, since she needed to stay available for more immediate concerns.
Of course there was that time when Cinderella's evil stepmother, Lady Tremaine, got hold of her wand to go back in time just to take away Cinderella's happy ending. At that time, the Devils helped Lady Tremaine, just so that they can get rid of the Fairy Godmother forever and to get the client into power. Thankfully, the Fairy Godmother was restored and it all ended well