What does Mandy confess in Chapter 4? Why did Mandy keep her identity a secret?

In Chapter 4, Mandy confesses to Ella that she is her fairy godmother. Ella does not believe her, so to prove it, Mandy magically cleans a up a bowl from the floor. Mandy explains that she has been godmother to Ella's family for generations, and keeps it a secret because fairy magic scares some people, and because other people would be asking her for favors.