Why does Anne begin a diary?

Her dad gave her the diary for her 13th birthday. She wrote in it every few days during the time they were hiding. Everyone says if she survived she would publish the book which came true that's why she didn't write the people she was hiding with their real name. But mostly what the diary (which Anne called "Kitty") was to her was a best friend - she could tell her diary anything she was thinking, as she had no one else in her life she trusted enough to do that.