A pen name, or pseudonym, is just a name you choose for yourself that will allow you some privacy (if you become famous), some anonymity (if you cross over to other genres or want to submit to a different genre, or even write in a racy genre--i.e. erotica), and/or credibility (if the agency thinks it would add some clout to your writing, in a sense) This last one, is sometimes the case when women write stereotyped "men's" writing (not so much of a problem any more, I think, but I'm not sure. S.E. Hinton, a female writer, wrote The Outsiders, which was about gangs and turf wars and her agent made her use a pseudonym to gain more readership)