Why biographies? They are a unique way to teach kids about invention, creativity, ingenuity, fame, power, and gifts of all sorts. Biographies offer people to relate to, real life examples of success, and a different approach to learning history. Giving the student freedom to pick the biography allows them to learn more about areas they are passionate about and discover some new areas of interest.
I do not pick the biographies. I bring the student to the biography section and make them select a variety of individuals across different disciplines. The only rule is the biographies must be varied and about someone you already do not know a lot about. It is fine if you pick an athlete, an artist, a scientist, an inventor and a politician. The point is to learn about a variety of disciplines through the story of a specific person. I also find that by requiring them to read so many biographies at once they start to discover the similarities of famous people across disciplines. I like encouraging inter-disciplinary thinking and studies whenever possible and reading a variety of biographies is one easy way to do so.
I find even as an adult I still enjoy reading biographies. It helps me understand some of the similarities and differences between the lives of famous people. I enjoy seeing the creative ways different people approach solving different problems and questions they face. I enjoy learning about the creativity and determination and discipline of many different people throughout history, both ancient and modern. I enjoy my kids discovering the same sorts of things for themselves.