DVD, HD-DVD, BluRay and other modern optical media might have the advantage of better longevity, improved picture quality and top-notch sound, but all many really care about is the bonus content that comes with their movie (aside from being able to watch these any time they want of course).
From trailers to commentaries to omake to documentaries to PC minigames to dull cast biographies that nobody reads, the modern DVD is often packed with bonus features, to the point that some have more minutes of footage in their DVD extras than they have in the film itself. Occasionally a bonus may be hidden as an Easter Egg.
This trend did not begin with DVDs — for years, video cassette releases would feature documentaries and alternate endings after the film's credits had rolled, though the more obscure LaserDisc videophile format's nonlinear abilities allowed the viewer to watch these snippets much easier, and also originated various interactive features like alternate cuts and commentary tracks. DVD's popular success finally heralded the advent of typical LaserDisc features for the masses.
Note that the lowest of the low in terms of bonus content is the "interactive menus", which really ought to come as standard: presumably the alternative is auto-play, but one still wonders what "non-interactive menus" would be: a list of scenes from an entirely different movie?note Only slightly above them are the "animated menus", which at least show a bit of effort was put into the whole thing, even if having to sit through the same animation over and over to navigate the menus is tooth-grindingly annoying.
DVDs are also a prime format to use for finding all those little in-jokes written on newspapers or signs that would normally be on screen for such a short time, you wouldn't be able to read otherwise. Although things like TiVo allow one to pause live video, too, before either pausing a VHS would cause the screen to get too blurry to read things.
Some early-release DVDs had what today seems like rather pitiful bonus content when the format was first taking off in the late 1990's and early 2000's, listing things like a text biography of each actor and a list of the films they had appeared in. Keep in mind this was before the rise of the IMDb, The Other Wiki, us and other online sites where people go these days to learn about actors and the works they have appeared in.
If the DVD has no bonus content it may be a Vanilla Edition - these may even have a "Bonus" menu option that just ends up being the theatrical trailer and Previews of other movies, though. Sometimes, the studio strips out the extras so they can encode the feature at a higher bit-rate so it'll have better visuals and audio. If it has mountains, it may be a Limited Special Collector's Ultimate Edition. If the bonus content is primarily scenes that were supposedly too hot for the cinema, it'll be an Unrated Edition.
Expect to see lots of Selling the Show.