One of the great advantages that DVDs have over VHS tapes is the number of special features or "extras" included on DVDs. In the old days a movie's theatrical trailer might be included on a VHS tape. If one was lucky, he or she might even get a "Making of..." documentary. This stands in contrast to DVDs, which sometimes have several more extras than one would get with the old VHS tapes. For the most part these extras are simply puff pieces, but in some cases one will get something more substantial and even educational.
The prime example of a DVD with extras of educational value is The Gangs of New York. Among the extras are a piece in which historian Luc Sante introduces the viewer to the Five Corners area of New York City as it was in the 19th Century, a glossary of the slang used in the area in the mid-19th century (taken from The Rogue's Lexicon published in 1859, and a Discovery Channel documentary called Uncovering the Real Gangs of New York. While none of these extras go into the depth that a good book or even a two hour documentary would on their subject, there is enough information there that the viewer could learn things he or she didn't know before.
Gods and Generals is another DVD with extras of some educational value. A piece on the autenticity of the film discusses the problems the filmmakers had in portraying the Battle of Fredericksburg. In doing so, the viewer actually learns a little bit about the battle. There is also a short documentary on Stonewall Jackson.
Of course, both Gangs of New York and Gods and Generals are based on history, but even films created from whole cloth (that is, not based on historical incidents) can have edcuational extras. The Others has a short documentary on xenoderma pigmentosum, the condition in which individuals absoutely cannot stand the light of day. A Hard Days Night has several extras which somestimes touch upon the history of The Beatles. Spider-Man includes a documentary on the history of the character.
With but few exceptions, I don't think anyone is going to learn a tremendous amount from DVD extras, but they might learn some things that they did not previously know. To me this is one of the wonderful things about DVDs. Of course, as to whether viewers actually take advantage of this and watch the docummentaries included with DVDs is another matter entirely...