These media complement news stories by providing another aspect to the news. In the case of photos, videos, and slideshows, that aspect is visual. In the case of audio clips, that aspect is audio. By having these multimedia, news organizations can deliver a bigger impact with their stories. A reader can see the wreckage of a car crash or hear the president's voice when he's giving a speech for example.
The type of writing that accompanies multimedia is simplistic and too the point. The first section usually describes what's in the photo or video, and the second section describes its importance. The writing is an important aspect of the media because if clarifies exactly what the media is disclosing.