In "Three Aunties," retold by Tim J. Myers and illustrated by Anna Bron, the trolls help the princess for a few reasons that often relate to themes of kindness, redemption, and the breaking of stereotypes. Typically in stories featuring trolls, they are depicted as misunderstood creatures rather than purely antagonistic beings.
In many versions of such tales, the princess's kindness towards the trolls or her boldness in facing challenges can lead to a change of heart in the trolls, prompting them to help her. This can demonstrate the importance of looking beyond appearances and preconceived notions.
If you have a specific passage or context from the book that you'd like to discuss further, please share it!