To hide their identity. I have a couple of friends who write and use pen names. Reasons vary: They work in the public eye and don’t want their real life audience to know about what they write. They don’t want their current employer to know what they write. They don’t want their friends to know they write. They want to write as a male when they are female, or vice versa, because they feel it will be better revived. I always thought if I wrote a book I would use a pen name.