Transparent parent classes to extend base class vs "normal" extension of a base class

I'm new to the Oxid framework, which makes an extensive use of transparent parent classes to extend a base class. I don't see the advantage of this technique. So I'm curious, is this a pattern? Where are the benefits?

Cheers, Kathrin

Example code:

\\ a class by the framework I like to extend
class A { ... }

Than the common way of extending this class by the Framework:

class B extend class B_parent

Where B_parent is class created by the framework via a config file, which looks like:

'extend'    => array(
        'A'    => 'path_to/B')