In vanilla LoD, once you beat Hell Baal, the game loop becomes "farm boss X for gear Y, then farm boss Z for runes." The Horadric Resonance system adds a parallel progression path: weapons and armor gradually "remember" which demons they've slain, allowing you to attune a single item to a specific Act Boss's "essence."

(A late-game layer of character customization tied to item "memory" and boss "echoes")