You know how they like to release more than one version of a film on DVD, like a special extended edition, commemorative anniversary re-releases, director's cut versions, etc? It's to trap you into buying the same movie again and again. You can save money by using Netflix for the basic movie content. If you want the special features, or packaging, or director's cut, whatever, then you buy it.
Netflix does carry a few director's cut versions, special editions, and extended editions.