When I wrote up a couple levels for Dragon's Delve, I walked into a
dungeon that already had a complete backstory and mythology in place. I didn't
have to answer any big-picture questions about why the dungeon--or the
PCs--were there, and I didn't have to justify anything to the players who would
be using that dungeon. All I had to do was come up with a set of cool
encounters that fit the existing outline and purpose and that offered a variety
of interesting challenges.