What was the purpose of writing the novel?

According to Louis Lowry herself, the idea for this novel was inspired by the history of Nazi-occupied Denmark as it was narrated to Lowry by her Danish friend, Annelise. Annelise had been a child when the events happened. Lowry admits that the story is set in a time and culture that is different than her own, but the idea is to show the value of humanity and the role that humans play in moments of adversity.
Essentially, Lowry wanted to inform and entertain at the same time. She tells a historical fiction story based on true events that happened to her friend, as well as to other characters who are immortalized in the novel. However, rather than dwelling entirely on the historical nature of the novel, she extrapolates the value of human agency, emphasizing on what happens when people see others in danger.
Moreover, she further expands upon the topics of empathy, human connection, mercy, and industry. Notice that, in this novel, people who would not have to worry about the Nazis persecuting them still put their lives in peril to help those who are in danger of being captured by the Nazis. Those who empathize with the persecuted plan, execute, initiate, and hope, all on behalf of the Jewish people, and not themselves. This is quite powerful!
Notice also that some feelings, such as empathy, sympathy, and industry are felt differently from person to person. Number the Stars, however,shows how, when people get together for a common cause, those feelings become stronger and focused on a purpose. The purpose is to help others battle the injustice of an evil regime. Telling this from the point of view of a child shows these emotions from an even more powerful, innocent, and unbiased perspective.
https://www.scholastic.com/teachers/articles/teaching-content/number-stars-authors-note/