I use film during shooting sessions for two reasons: First, it lets players see their shot and any needed corrections.  This is an extremely valuable tool, especially for younger players.  Second, when I work with advanced shooters, I often need slow motion film to even identify subtle mistakes or relapses. Often times all you can see real time is the result of the mistake (i.e. bad or no ball rotation, lateral release movement) but not the underlying cause (i.e. poor hand placement, wrong release point, inconsistent leg power, etc.).