The first requirement for Freudian clinicians using a personality assessment tool is some sort of road into the unconscious, to track residue from early childhood experiences and reveal hidden conflicts and impulses.
Given that Freud deal with the unconscious and the subconscious a lot, and overall with the conscious, his followers also need to be very careful when they want to assess these more delicate parts of our minds and psyche. They need to know what they are doing so as not to mess up a person's life.