They can evoke emotions. Photographic images can tell complete stories. They can supplement and extend description. They can convey a sense of immediacy and proximity. They too can evoke emotions.
Animation provides visual stimulation, actively engaging the eyes in the experience.
They can be used very effectively to demonstrate processes or clarify complex relationships. Audio clips can convey affect more readily than text can do. Sound can provide a sense of immediacy or proximity. Again, the medium can evoke emotions through associations. Video clips have the combined advantages of still images and audio.