Why did North Korea just detonate a nuclear bomb?

Yesterday saw the first nuclear test since 2009 and North Korea's largest detonation to date, over six kilotons of force generated from an underground site 20 miles west of the town of Hoemun. It's a powerful reminder that the country is still nuclear, and still dangerous. But for all the diplomatic damage they've incurred, it's hard to say what they've gotten back. In fact, it can be hard to say why the test happened at all. For a country that's already known to have nuclear capability, what go...