First of all, it is because the era in which The Book of Songs is located is far away from now, and there are many contents about etiquette system and history in The Book of Songs. So time can be studied as historical materials.
Du Fu is a realistic poet, and his poems are known as "the history of poetry", which shows that everyone admires his poems. He is different from Li Bai's romanticism, and most of his poems are said to reflect the current situation. Du Fu's personal emotional expression of the status quo is very real. This is also the main reason why Du Fu's poems can impress people.
Because he was not in the heyday of the Tang Dynasty, he experienced the Anshi Rebellion. After that, the Tang Dynasty became less prosperous and gradually declined. Du Fu personally experienced that era. Maybe some of his poems look easy to understand, but they can hit people's hearts directly.
Because many of his poems recorded the social situation at that time, the cruel scenes of the war, and his personal political ambitions were frustrated. Du Fu wrote many kinds. He almost recorded a period with his poems.
His "Three Officials and Three Farewells" reflects social unrest by describing people's lives, so it can be a history of poetry.
Therefore, both the Book of Songs and Du Fu's works can be studied as historical materials for different reasons. One is that there were not many historical materials that could be studied at that time, so its content is also worth studying. The other is because his works vividly describe the historical background and social status quo at that time.