bcrypt and other modern hashing algorithms allow strengthening your hashes (by computing more iterations) at a later date should technological advances make your current number of rounds too weak. This allows you to 'upgrade' your password hashes from 20 rounds to 20,000 without needing the user to re-enter their password. If you publish your hashes at any point, you lose the benefits of this, if your hashes become weakened by technological progress you need to reset your users' passwords.