Using a video in class is NOT like watching sports or drama or sitcoms at home. Television can be a powerful educational tool, but if we want our students to absorb specific content from what they see, we need to give them a model for viewing that is active and critical. However video can help accomplish things you can't do any other way.
TV does the following very well.
Spark interest in a new subject area. Imaginative and quick paced video can inspire your students to pursue a subject. Enrich content by demonstrating new applications or insights. Practice a skill such as note taking, problem solving, predicting, listening, etc. The TV can't act as teacher if students aren't active.