In his latest post Rob Allen seeks to answer the question "what problem does dependency injection solve?":

Zend Framework 2 comes with a dependency injection container (DIC), as does Symfony 2 and Aura, along with many other PHP frameworks that target PHP 5.3 or hight nowadays. This article attempts to explore the problem that a DIC tries to solve.

He gives an example (somewhat based on the same structure of his Zend Framework tutorial) showing how you could use DI to inject the Artist object into the Album object. This allows for more flexibility if more Artist types are added (extending the main Artist, of course). His second example shows how to use a DIC to hold album information.