Live theater can be a magical experience. Each performance is unique and, unlike recorded entertainment, the live audience is an integral part of the event. The performers on stage feed off the energy of the people watching them -- the laughter, the applause, the gasps of surprise, even the occasional heckles.
Watching a performance on video, while a great way to chronicle that once in a lifetime experience, still can never fully capture the sensory delights of that live moment in time. Sure, with proper production values, it can come close, but it can never equal being there in person.
A photograph, on the other hand, creates something new. It captures an instant, a brief little fragment of what people witnessed. Many times, that image becomes a memorable representation of a particular production, joining other recollections of what took place.
A good photographer will frame just the right second, create a sense of anticipation, tell a story, freeze an action so that it represents the performance. That's the difference, it doesn't attempt to recreate the moment as video sometimes seems to be trying to do. In the right photographic hands, it aims to capture the essence of that performance to be remembered.