What the title says.
There is no great sword & sorcery adventure without exploration and discovery. Exploration requires wilderness, and wilderness implies a negation of the World-Building (“everything needs to be mapped and known”) ethos in sff. Adventures require tension, and tension is impossible if everything is mapped, if civilization is dominant, or if the protagonist can go back to their HQ to resupply and rest. Hence, a vast blank space on the map and danger from wich you cannot escape are required: dungeons, caves, ruins, or being trapped inside a magician’s tower. The hero, if he wants to survive, has no other option but to keep going down.
The hero must prove his worth, which means that technology and gadgets should frown upon, and help and social support/assistance should be kept to a minimum. A half-naked barbarian is, for those reasons, a strong symbol of raw strength. On the other hand, if the protagonist can appeal to the law, the army, or other official and socially legitimated sources of force, the whole point of heroism disappears. Barbarians, rogues, criminals, wanderers, and travelers are the natural heroes for this genre.
But those dangerous places where the hero is trapped need to be inhabited, and the nature of those creatures must be somewhat elusive or even unknown to maintain the sense of discovery and tension. Therefore, unlike other genres of fantasy where you design Races, with their history, culture, “nations,” or domains (“Orcs live here,”) in Sword & Sorcery you design Monsters, which may be one of a kind for all we know and their most important quality and function is that they are single-minded predators.
The existence of ruins and abandoned places implies previous civilizations that don’t exist anymore, and the existence of monsters in those ruins means that they have not been visited for a long time and that there are no forces of law that can bring order to the wilderness. Otherwise, there would be no need for adventures, and there would be no more treasure and ancient artifacts to discover.
Therefore, the best of Sword & Sorcery requires an almost post-apocalyptic setting, with a depopulated world built on top of the ruins of an Atlantean-like Golden Age, now with a whole world to explore and “Here be dragons” at every turn. The reason people have not realized this is because, unlike other post-apocalyptic stories, Sword & Sorcery is a genre of force and not despair, and it does not dwell in the past and its destruction but in the present, even if it’s a deadly present. But deadly conditions are needed if the heroes want to prove their worth.