This interesting question is a little vague. Do you mean: rather than rent them, rather than go to theaters, rather than spending money on other luxuries?
Here's why this movie lover collects, in no particular order:
-astonishing quality. Beats any other format hands down, especially with a decent HT, which I'm lucky enough to be able to afford.
-Form factor convenience. They play on my notebook for travel and fit in plastic CD sleeves, but also take my home theater to the limit.
-lendability. Lots of folks are just starting off their DVD collections. I help them get the most out of their players.
-interesting extra features.
-trading/resale potential. I sell ones I grow tired of for more than I paid for them, with very little effort. Trading is easy too.
-Packaging. Laugh if you like, but I like having the multi-page booklet inserts, nicely labled keep-cases, etc.
-ready access to a movie library. I really like being able to recall a scene from a great flick, and then simply pulling it out to view.
-far more convenient then renting. No getting the films, no returns, no waiting for netflix, hoping they have what you want.
-I like supporting great products. When an artist makes a great film, or a studio produces a reasonably priced, well-done DVD, I like endorsing them with my pocketbook.
-english subtitles, baby. My hearing is fine, but I miss plenty of dialogue in a noisy movie. Not with most DVDs.
I have around 500 DVDs. The first 200 or so were purchased in the good old days of dotcom maina, where you could get them for next to nothing using brandsforless/coupons/rebates etc. Now, too keep my costs down, I buy primarily columbia house memberships and good condition blockbuster previously owned DVDs. My average price for DVDs is around $8-9, including box sets. (This average has crept up some over the last year.)
In short, because DVDs offer superb entertainment value for the $, and are highly "liquid" (tradable/sellable), they are the only thing I find worth collecting other than books. (I've never purchased a VHS tape.)