A key attribute of modules is encapsulation. The objects that are not exported from the module are private to the module, and cannot be accessed from code outside the module. To reiterate, modules are treated as if they were written as follows:
(function() { ... contents of module file ... })();
This JavaScript idiom defines an anonymous private scope. Anything declared within that scope cannot be accessed by code outside the scope. That is, unless some code makes object references available to other code outside this private scope. That's what the module.exports object does: it is a mechanism for the module author to expose object references from the module. Other code can then access resources inside ...
