Why calling next() in express routes is optional?

In many examples of Nodejs/Express, I see that calling next() is optional in case of success.

exports.postLogin = (req, res, next) => {
  passport.authenticate('local', (err, user, info) => {
    if (err) {
        return next(err);
    }
    req.logIn(user, (err) => {
      if (err) { return next(err); }
      req.flash('success', { msg: 'Success! You are logged in.' });
      res.redirect(req.session.returnTo || '/');
    });
  })(req, res, next);
};

Also, it is easy to skip callback next in the args:

exports.postLogin = (req, res) => {
  res.render('some-template', locals);
}

If I compare this with async lib or typical Javascript's asynchronous model, a missing callback will not give data or halt the process.

What does express do to take care of control flow when next is not called?

via ankitjaininfo