This question refers to Anne Frank's diary, known most often in English as The Diary of a Young Girl.
In this diary, Anne Frank describes the experiences that she faced while she was in hiding with her family during World War II. This allows the reader to learn about the everyday details of the life of a person in this situation, which is not possible to learn from a secondary source. Secondary sources are more likely to give an overview of the general situation, while the diary gives us a specific description of a single life. Moreover, the diary includes details that the secondary source rarely includes, such as descriptions of the emotions that existed in the people suffering through the Holocaust. The diary can teach readers about the ways in which people overcame their problems, and how they were able to find hope and strength despite their suffering. This creates an emotional link with the characters that is rarely developed in secondary sources.