I think this was because it was thought that a child using just the first name to an adult was disrespectful but the full title “Mrs Smith” would be too formal for a child to say.
可能依家唔興以前興?
... Later also as a term for an unrelated female family friend... As a more or less informal term for any woman... Auntie is now used as a general term of respect for (usually older) women in various contexts in many varieties of English.