The achievements are a great idea really, the completists know for sure if they have 100% a game, people can set little goals for themselves, share their game score for obvious e-penis points, and it encourages games to make a lot little things that give positive feedback in the game.
Of course, it also encourages some games to put batshit achievements in them that you couldn’t do with an offical guide or an unofficial guide on GameFAQ’s lifted straight from an official guide (hello Blue Dragon!).