Well it certainly isn't very clear why a dwarven settlement would want a monsters-and-treasure dungeon lurking just below its living quarters, so either you could try to reason out what a "dungeon master" would do in a sensibly thought-out fortress/civilization, or you could presume the existence of a dungeon master that controls a dungeon and concoct rationalizations from there.
Going from the second premise, some possibilities:
- as I suggested before, dwarves attach cultural importance to dangerous dungeons, and your fort gains prestige depending on the value of your dungeon
- the dungeon is where they keep their coins, when the dwarven economy returns
- the dungeon master does !!SCIENCE!! on exotic creatures; he not only tames them, but develops extracts from them, perhaps gleans secrets from them
- you actually need a dungeon master to organize expeditions into the subterranean caverns, because civilian dwarves are actually (and sensibly) scared of unexplored caves. The dungeon master's function is to map the underground and corral the beasts therein. Also, you have to guard him, because you can't appoint dungeon masters, they come in immigrant waves.