One effect of the novel’s use of a first-person narrator is that

One effect of the novel’s use of a first-person narrator is that it gives an insight about narrator's thoughts and feelings.

Explanation:

When a novel is written with the point of view of a first person, the reader comprehends the story from the narrator's perspective.

So it becomes easier for the reader to understand the feelings of what the writer had tried to convey to his readers.

it not only provides the outer perspective but also gives an insight on narrator's thoughts and feelings.

because most jews were in the concentration camps and dying horrible deaths while anne could be with her family in a warm place with shelter, food, water, and a place to sleep.