express框架, 为什么会混合使用内置的 http(s) 模块？

var express = require("express");
var app = express();
app.use(express.static('public'));
app.use((request, response, next) => {
    console.log(request.method + " " + request.url);
    next();
});
app.get("*", (request, response, next) => {
    response.end(request.url);
});
app.listen(80, function () { console.log('http://localhost:%s', this.address().port) });

var http = require("http");
var app = http.createServer(function (request, response) {
    console.log(request.method + " " + request.url);
    response.writeHead(200, { "Content-Type": "text/plain" });
    response.end(request.url);
});
app.listen(80, () => console.log('http://localhost:%s', app.address().port));

let express = require("express");
let _express = express();
_express.use(express.static('public'));
_express.use((request, response, next) => {
    console.log(request.method + " " + request.url);
    next();
});
_express.get("*", (request, response, next) => {
    response.end(request.url);
});
// _express.listen(80, function(){console.log('http://localhost:%s', this.address().port)});
let http = require("http");
let httpServer = http.createServer(_express);
httpServer.listen(80, () => console.log('http://localhost:%s', httpServer.address().port));

// ...
_express.listen(80, function(){console.log('http://localhost:%s', this.address().port)});
// ...
httpServer.listen(81, () => console.log('http://localhost:%s', httpServer.address().port));

/**
 * Listen for connections.
 *
 * A node `http.Server` is returned, with this
 * application (which is a `Function`) as its
 * callback. If you wish to create both an HTTP
 * and HTTPS server you may do so with the "http"
 * and "https" modules as shown here:
 *
 *    var http = require('http')
 *      , https = require('https')
 *      , express = require('express')
 *      , app = express();
 *
 *    http.createServer(app).listen(80);
 *    https.createServer({ ... }, app).listen(443);
 *
 * @return {http.Server}
 * @public
 */

app.listen = function listen() {
  var server = http.createServer(this);
  return server.listen.apply(server, arguments);
};

let express = require("express");
let _express = express();
_express.use(express.static('public'));
_express.use((request, response, next) => {
    console.log(request.method + " " + request.url);
    next();
});
_express.get("*", (request, response, next) => {
    response.end(request.url);
});
_express.listen(80, function () { console.log('http://localhost:%s', this.address().port) });
let http = require("http");
let httpServer = http.createServer(_express);
httpServer.listen(81, () => console.log('http://localhost:%s', httpServer.address().port));
let https = require('https');
let fs = require('fs');
let httpsServer = https.createServer({
    key: fs.readFileSync('./cert/cert-key.pem'),
    cert: fs.readFileSync('./cert/cert.pem')
}, _express);
httpsServer.listen(82, () => console.log('https://localhost:%s', httpsServer.address().port));