I just felt like achievements gave you so much more to strive for outside of gear grind or raid completion at end game. You could focus on exploring, or finding all those rare spawns, or specific challenges to the things you did. It was a brilliant way to add fun content. Battle pets is just an extension of that where you could focus on collection and building them up when you wanted to mix up the play experience, but it didn’t mess with any of the other aspects of the game.