Photography is to me a tool of storytelling, of style, and of time.
Photography blurs the line between reality and fabrication. By drawing a frame around the world you become master of your audience's attention and can craft stories of fragments of reality.
If design is when an idea, an abstraction, leaves someone's mind and becomes part of the physical world then style is when the physical becomes abstracted. A photograph is an artifact that of abstraction and might offer only subtle interpretative hints if the photographer chooses.
Finally, a photograph communicates time—even better than video, really. Freezing just one moment forces us to be aware of what came before and after. We are not informed of these things and thus are made to bring time to the experience ourselves. Even a series of snapshots on a contact sheet require the viewer to imagine the moments in between
Thus to me a photo is best when it is striking, has emotional energy that connects its singular moment to the potential moments before and after, and leaves me with a new idea. I want to strive to achieve this in my photography. I have a long way to go.