Just what is it in there that the PCs need so badly? How does it affect the greater ongoing plot?
In short: the mcguffin. It sounds pat, I know. But the reason to play DnD (and all the other games) is to have some fun. The dungeon crawl is one of those things that people enjoy doing. Mostly I don't use them but every now and then a mindless hack'n'slash is great fun. ANd to get people into the dungeon you need a mcguffin. If that's one that the players can really get behind (whatever it may be) then all the better. If it's one the players come up with themselves that's best!!
And as several folks have pointed out there's a variety of ways to emmulate a dungeon crawl that are more realistic than an enormous serious of perfect 10'x10' corridors filled with wandering monsters. I nearly typed "wondering monsters." Perhaps they are wondering why they're in an enormous dungeon. But I digress. Aw heck, another brief digression: In some cases perhaps they are wondering why they happen to be perfectly 10'x10' themselves? Which begs the question: Do oozes dream of gelatinous sheep?
I think the dungeon may have been chosen originally (and there are people who can correct me if I'm wrong) for several reasons: the ease of constraining the players in a situation that has been prepared. (that old chestnut of the DM has put a lot of work into the game and quite naturally s/he wants the players to follow through on it.) Possibly for the mythic feel of "entering the labyrinth." The whole Joseph Campbell thing.
If you're asking for specific examples I've used, mmmm. let's see if I can remember a few.
The castellan of the keep offers a reward for any information about the nearby caves that are infested with humanoids.
The PCs have unintentionally freed a powerful efreet and must find the means to combat it within the lost tomb of the wizard who had imprisoned it in the first place.
The DM says: I've just bought this crazy module called
Tomb of Horrors, it'll be a hoot. PS: anyone says this to you, save yourself the effort. It's a hoot all right, just not for the players.
If you want ones I've come up with:
a prospector has gone missing. They were last seen in this area. (this one was suppsoed to work on PC guilt too on account of the prospector being in the pay of a PC. Alas that character retired between me drawing up the dungeon and the dungeon getting played.)
OK I admit it: the last few paragraphs Ivem just been writing to get my own thoughts out there.