A) They make structuring adventures easy
As a designer (pen-and-paper or computer/console game designer), the confining nature of a dungeon makes things a bit easier to structure. Dungeons are not open-ended environments, and form natural barriers requiring predictable navigation. This means designers don’t have to jump through all kinds of contortions to structure an interesting adventure without imposing arbitrary limits on players. Dungeons form natural tree structures (at least well-designed ones do), albeit with loops, which lends itself easily to creating ordered events, choke points, and so forth.
Of course, in many games the players can take some extraordinary measures to bypass them, like spells to pass through walls or teleport. This is very cool if the adventure is structured flexibly enough to tolerate this -it rewards the player for taking initiative. But it’s still an extraordinary event, not the preferred means of navigation. Whereas in an outdoor environment, it’s harder to justify why the player can’t just take a shortcut through the woods to go straight to the next castle.
