& get_instance() vs $this

When you use =&get_instance() you pass object by reference. Thats mean you don't create a new object, you only operate on existing object.
You MUST use get_instance() if u want model object or library object in for example another library. In controller you extend CI_controller, in __construct() it use get_instance() so you have access to created instances of model, libraries or something else.