Your research is limited to the scope of documents available at the time of your research. If you happen to find a biography that cites even older sources of information, this perpetuates your search. This biography may have included information from documents that have long since been lost or destroyed. Millions of records have been lost in fires, floods and other catastrophes. This secondary research essentially becomes a primary source, especially when original documentation can not be obtained.
Biographies make for interesting reading.
Even if they are an embellished perspective of a person or family, they still often give you a lot of details that you could not obtain from any other source. They also give you a story based reference beyond the dry facts of douments. Think of the biographies that you could write about your family. Add to the references for the future researcher. Gather the information about your timeline.