What's the point in the Achievements? What are they for?
You can screenshot them and show them off to future generations when Duolingo has evolved into something entirely different.
for those who use Duo for more of a game than as a language learning site, they encourage them to keep going. as another commenter said, they're a form of motivation. they make the site more fun. they give the learner something to work towards. they're most aimed at younger ppl I would suppose, those who need some motivation to do something educational.