Dragons are incredibly dangerous threats, but their very popularity robs them of suspense. They fly, they have breath weapons, and you don’t want to get caught next to one when it full attacks.
The idea behind gruesome dragons is to make them more interesting for players and GMs. Rather than face yet another color-coded dragon encounter, the players find themselves facing a plague dragon capable of ending civilizations, or skittering draconic ambush predators obsessed with building the perfect adventurer trap. Gruesome dragons have uncanny appearances, smells, and sounds that makes them more frightening to even veteran characters.