After all, that's the question developers are supposed to ask themselves. See, that's called improvement by experience. All beings, barring protagonists of conspiracy TV series, are supposed to gather knowledge from previous experiments and apply the empirical data so the same mistakes will not be repeated. I speak about contorted corridors that twist throughout the edification, leading to randomly placed rooms of poorly defined purposes, if any. Any compound that does not hold an explicit justification for its existence is an invitation to question - to step out of the character's eyes and judge the cold, schematic merchandise.
Immersion is golden.
Believability is golden. Those of us who played Daggerfall learned this lesson the hard way. Their famous 1001 dungeons were randomly generated. Sometimes in the night, I still see myself running aimlessly through the askew corridors and mirrored rooms, after giving up on the seizure-inducing 3D map. Also, the dungeons should all have some sort of story.