There are three main reasons for this:

1. To make its falsehood known: Muḥammad ibn Rāfiʿ says, “I saw Aḥmad ibn Ḥanbal in the gathering of Yazīd ibn Hārun with the book of (Zuhayr from Jābir al-Juʿfī— a liar) I said to him: ‘Why do you prevent us from Jābir al-Juʿfī but write from him yourself?’ He replied: ‘So that we may recognise him (as a fabricator and thus be able to distinguish his fabricated ḥadīth from the sound ones).'”…