I've always found the slime trope weirdly comforting in dungeon crawls. They're this baseline mob that sets the tone—if you can't handle a basic corrosive blob, you have no business delving deeper. But the 'demon' twist adds this layer of forbidden alchemy or corrupted nature.
In something like 'Dungeon Meshi', slimes are almost ecosystem engineers, cleaning floors and recycling waste. Make them demonic, though, and suddenly they're not janitors; they're the dungeon's immune system, actively hostile and mutating. Their role shifts from a tutorial enemy to a signifier that the dungeon itself is alive, malignant, and maybe even sentient.
It's less about the combat challenge and more about the atmosphere. A gelatinous demon oozing through cracks tells you the rules of reality are broken here.