In development environments, objects are very often dependent upon other objects. For example, content elements are dependent upon pages. This means that when we instantiate a content element, we also need to instantiate the dependent page object. This happens through a constructor on the dependent object, which may also control further initialisation objects. In our example of a Page, this could be a page tree or a rootline object.
Because of these fixed dependencies, you would have formerly run into problems when you tried to test individual methods within the page object - without a related page tree object, you couldn't run your tests. This problem has been overcome through the implementation of dependency injections. Many constructors have been removed or re-written for Extbase, and relationships to other classes moved out into smaller, dedicated methods. This means that an object can be instantiated without always needing to instantiate the dependent objects.
There is an additional advantage when it comes to phpUnit: you have the ability to bind a completely separate object to your page tree. For example, if you're working with a database object, you can instruct phpUnit to bind a database object which reads from and writes to RAM-based tables. This allows you to run tests on your system without running into the danger of accessing and potentially corrupting “real” data.