It's just a fact that people sometimes are interested in artists as well as or more than the art they make. The not-so-great art may not be of interest to you, or some particular artist may not want to release it, but it seems to me that probably some people want something more comprehensive for similar reasons that they may read biographies.
I'm indifferent to Michelangelo's art, but I have read The Agony and the Ecstasy. In this particular case, a less filtered view of his art doesn't disillusion me because I'm not especially crazy about any of it. But he was famous, influential, and technically capable, so as a person was of interest to me.
