Why does accessing the session object make Express-Session work?

app.use((req, res, next) => {
    req.session.init = "init";
    next();
});

require('dotenv').config();
const express = require('express');
const session = require('express-session');
const MySQLStore = require('express-mysql-session')(session);

const app = express();

const options = {
    host: process.env.DB_HOST,
    port: process.env.DB_PORT,
    user: process.env.DB_USER,
    password: process.env.DB_PASSWORD,
    database: process.env.DB_DATABASE
};

const sessionStore = new MySQLStore(options);

app.use(session({
  name: process.env.SESSION_NAME,
  secret: process.env.SESSION_SECRETS,
  store: sessionStore,
  resave: false,
  saveUninitialized: false,
  cookie: {
    secure: false,
    httpOnly: true,
    sameSite: 'strict',
    maxAge:600000
  }
}));
app.use((req, res, next) => {
    req.session.init = "init";
    next();
});

require('./startup/headers')(app);
require('./startup/routes')(app);

app.use((error, req, res, next) => {

  const status = error.statusCode || 500;

  res.status(status).json({ message: `Error`});
});

module.exports = app;