I'm not sue if this is the real answer but................................
I do this after the fish has taken the fly and is already on the hook. It either pulls a poor hooking lose or sinks it in so that the fish will be landed. As others have said this is primarily used when fishing streamers in salt or fresh water. When fishing dries I do virtually nothing and the fish hook themselves when they turn to the bottom after taking a floating fly.
it also allows for a hook set when the rod and or you are not in an optimum position for a twitch of the rod. Actually, once you become accustomed to a deliberate pull on the line rather than a reflexed jerk of the rod I believe you hook more fish.
Might as well do it, everyone else is