I agree with Brandon on its application as a training aid. You can watch the student's light bounce up and down if they're flinching. You can observe the steadiness of their trigger pull - especially with a long double action pull.
There are some instances where the intimidation factor has deescalated an incident. It's worked on the streets and in some prison fights (too bad).
While they're nice, they're no substitute for practice and concentration on your front sight. I like mine (yes, I own one for a J frame), but I keep a 9mm and 45 for home defense.
They are fun to play with and watch the dog chase after too.