Some people create pseudonyms as a platform for trolling or propagandizing without risking their main account, other people simply morally object to having a consistent online identity on principle, or want to say something controversial or confidential to protect their main account or identity. In every case, the pseudonym is an attempt to protect one's identity, and the person making it considers it necessary.
They are usually discouraged, downvoted and flagged as they should be. But that happens with established accounts as well, so the anonymity itself doesn't seem to be the problem in those cases. Unless your account links to your real world identity in some way, there isn't much value in having an established account at all.
The corpus of postings by any consistenly used pseudonymous account can probably be fingerprinted and linked to a real-world identity if someone cared enough.
E.e. google probably could do it as they have every comment on this site and also have all of your email.
