In what ways is a diary a unique form of literature? How does a diary compare to a novel? Why do people write diaries? Why would a diary be particularly helpful to someone in hiding?

I can make a general comment here:

Being a thirteen year old girl, Anne wrote her diary for the same reasons any adolescent girl might write a diary. She wanted to chronicle her feelings and make sense of the world around her. Of course Anne was living in a pressure cooker. Hiding from Nazis with so many people in a small annex was not easy. Ann used her diary as a therapy to cope with not only teenaged angst but to keep perspective and her sanity in such a close environment. The original diary wasn't to just "Kitty" but to many pseudonyms. The diary became a sounding board for Anne and, eventually, a passionate piece of historic evidence for the world.