Dungeons and raids have been core to World of Warcraft's endgame progression for most of the 20 years it's been around. You level up, you run dungeons, and then you do raids. There have been a few other ways of getting loot, but to get the best loot, you had to group up.
The War Within introduced Delves, mini dungeons scattered throughout the world that are designed for solo players. They scale up in difficulty and reward items that are on par with the kind of gear you previously could only find in group activities. Delves were destined to shake up WoW's endgame progression, but egregious bugs with enemy damage and health scaling early in the expansion had players questioning their purpose.
