Well a lot of coursera courses come with :

- a community where you can discuss courses like in a real class

- exercises,source,... that are often reviewed by real teachers or peer-review which absolutely invaluable at least for me . I learned a whole lot because of this.

So it's way more than a youtube channel with a bunch of videos. Course are shot specifically for coursera.

Depends on your preferred learning model. If you can learn by just absorbing the narrative lectures, fine, but not everybody prefers it. For some, like myself, a degree of structure and interactivity provided by Coursera, helps a lot.