callback express node-async node.js

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?