They’re not “for” anything. You can’t trade them in for a commuter coffee mug or a T-shirt. Think of them sort of like getting stickers on your class assignment. They don’t have any value and they don’t mean anything, but people like them anyway.
They’re awarded for “great questions” (GQ) and “great answers” (GAs) and also for signing in on consecutive days, as this explains.
I look at them as a rough indicator of how long somebody’s been around and how much they’ve contributed. They’re of very limited significance, both because only the first x points from a given jelly count in your lurve score (the rest are blanks) and because we have far fewer members these days. It takes a long while for an old jelly to move up by 100.
But they’re part of the system, and this is the system we have, and it’s no longer in development, so wysiwig—and no use asking for changes. I’d say most of us like things as they are, or we wouldn’t stick around.