One possible good use of badges/cheevos/etc is to be a guide. If you're starting as a blank slate and wondering what do I do here?, having a set of neatly defined goals can give that void some structure.
That being said, the achievements on a platform often reflect the goals of the platform. If you get a badge for having a certain number of followers or logging on a certain number of days in a row, that shows what the platform is trying to guide you to do. It's reflective in many ways of the site's culture if, say, you get a gold star for having a certain number of followers vs getting a gold star for posting the best comment of the week.
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