Well, mostly, some readers like biographies because it's they're style of reading. For example, most readers like chapter books, or graphic novals. But biographies usually tell about that person, or something they did that made a huge change in life. And they might also inspire them, to keep reading, or on their quote.
So, some readers like biographies because they might like learning about other's lifestyle, and to inspire.
Hope this helps---Aparri