The idea is to better understand how modules work inside node!
First of all its very important to remember that we use module.exports that is an object so we can actually do this:

var users = {
 num: 10
};
module.exports = users;

This will fill the module.exports object with the object that we want to use, but there is an important point using this method! Since it’s an object and in Javascript when you assign an object you are actually passing the reference in memory, we can do stuff like that:

//multiusers.js
var users = {
 num: 10
};
module.exports = users;

//index.js
var users = require("./personal_modules/multiusers");
console.log(users.num);
users.num += 10;
console.log(users.num);//20
var users = require("./personal_modules/multiusers");
console.log(users.num);//20

10 20 20

So in this case we are actually working on the object and its value in our memory, so if we change the value in the memory this will change for everyone that are using the app, while if we do like this:

//multiplayer.js
var multi = 10;
module.exports.multi = multi;

var multi = require("./personal_modules/multiplayer").multi;
console.log(multi);//10
multi += 10;
console.log(multi);//20
var multi = require("./personal_modules/multiplayer").multi;
console.log(multi);//10

10 20 10

And this makes a whole important difference when constructing the modules system!!