It does work, it helps build your cognitive feedback.it’s a memory and sensory game your brain has to play with itself kinda like the other guy was talking about. The thing is though, since you’re comfortable talking to yourself as most stutters are, eventually you need to ask someone to stand off to the side of your mirror (if you want them in frame that’s even better) because it puts you in a real life environment where you’re talking and someone else is capable of hearing and interacting. This is why you stutter in the mirror in front of the therapist but not when you’re alone it’s because the presence of another human causes you some sort of disturbance which is why if you’re doing this you should ask your parents,siblings, or someone close to you to just stand in proximity to you while you’re talking to yourself to simulate a real conversation.