Some people use DAF devices as a sort of "reminder" to attend to their speech. It can facilitate more conscientious awareness of how you are talking, and provide an impetus for physically modifying or channeling your stutter. For example, for people who have lots of secondary and struggle behaviors, hearing the DAF can cue them to not struggle/push so hard, greatly improving the ease of speech. The "magic" effect where it just makes you fluent without any conscious awareness on the speaker's part is often short-lived, as you said. The above approach (using it as an aid to maintain awareness and attention to how you stutter/speak) can yield consistent long-term results.