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.
Still, they should be used sparingly, and only when they convey the message better than a still image, animation or audio clip could do.