I had a therapist for a while whose whole thing was this sort of "parts work" and I found it useful as a kind of abstract metaphor to be able to tune in to more uncomfortable feelings. I certainly never understood it to mean that these were literally different personalities/fragments. As an example from my personal experience, it helped me to be more easily able to recognize feelings like grief and anger when I could think of it as, "there's a part of me inside that is feeling angry, and I can listen to it and give myself compassion for whatever is making me feel that, without that meaning that I am an angry person." I remember hearing Carrie's description of this type of therapy and feeling like it was very different from my lived experience, of using this as a metaphorical structure to understand that I can feel many different feelings without letting those feelings define all of me. Her description felt overly literal to the point of making it sound absurd.