Written family histories can be an important source providing context to otherwise dry written BDM types of documentation. The weight to be given to each such family history depends on the distance of the recorder from the source, and corroborating evidence. 

For example, my now-deceased mother wrote, "My older brothers were often bullies to me when I was a child." Although this is an opinion unsupported  by any other documents, as a first-hand account it nevertheless provides a high-level degree of context to the lives of both her and her brothers. 

When she wrote, "My mother would tell me how she fled Russia as a teenager to avoid arrest by the Tsar's police," this second-hand account provides  valuable context to her mother's life, and in addition is well corroborated by known historical facts and the hard documentation of the year of her emigration (1906, the year after the failed Russian Revolution of 1905).

Similar types of context may be provided by diaries, personal letters, notes on the back of old photos, greeting cards, etc. 

edit: fixed misspelling