Photography is very powerful for many reasons, one is that is shows what is happening, including all the details happening in a scene. Another is that it shows people what is going on without the need to read captions, its basically self explanatory.
An example is Lewis Hines, when he took photographs of child labor, and how children were treated in factories. Of coarse, it was not easy to get the pictures, but when he did, everyone was in shock. It showed the truth to what has happened.