promise & mocha: done() in before or not?

I am reading some tutorials on promise tests in mocha. There is a piece of codes:

before(function(done) {
  return Promise.resolve(save(article)).then(function() {
    done();
  });
});

Why done() called in the then() in the before()? What is the difference between the above codes and the following codes:

before(function(done) {
  return Promise.resolve(save(article));
});

Thanks

UPDATE

My question is to compare with the following codes:

before(function() {
  return Promise.resolve(save(article));
});

Sorry for the typo.