Confused About Objects and Classes in CodeIgniter?
I am not familiar with CodeIgnitier. But familiar with other PHP frameworks. Most of frameworks use this way for performance improvements, registering things, executing certain events, and making things simpler for developer...
For example if you want to create class "car" with is somewhere in library directory you would have to include the file first before you can create object of that class (miltiple lines of code, more room for error). The framework will create the class and includes related files in 1 line of code (easier and safer).
Framework way also works as a factory. Instead of recreating an object, it will create object only once and every time you call the method again it will return the reference to existing object.
More things are happening behind the scenes when you use framework. Things are getting registered, etc...
CI doesn't replace class behavior per se, it simply adds functionality that allows access to custom libraries/models/views as singleton objects via the core object for simplicity.
Nothing is stopping you from creating (as I have in one of my projects) additional files with classes for non-singleton entities and require them in a model for further use. On hindsight, I should probably have used helpers for this.
What the loader ($this->load) class does, among other things, is it creates a single object of the specified class (model, library or view - not helpers though, see below) and attaches it as a property of the core class that is normally accessible via $this.
Helpers are a bit different. They are not attached, but instead simply 'read' into the global namespace from the point where they are loaded.
To answer your question, it would be more proper to use the loader class in instances where you don't need more than one instance of a class created. If you need 'entity' classes, your best CI-compliant bet would be to create them as helpers.
Given only this context, this looks like Inversion of Control (maybe I'm wrong, I haven't looked too closely at CodeIgniter).
You don't want to rely on the type car as in new car(). What if later you want to make $var a racecar? $var can still do the same things, but it is forced to be a car because you constructed it directly. Or what if you are testing this class, but car is some complex object which calls some external service. You want to test your logic, but don't care if the car service isn't working. So you should be able to change $var to actually load a mockcar. You can't do that if you do $var = new car().