Why indeed. We understand the impulse to ask the question, and certainly look skeptically at anything that is trying to encourage us to use a given system because it is supposedly better. Well, we don't want to encourage anyone to use our framework, we aren't promoting it, and don't really care if anyone uses it. We use it, and that's enough for us.
That being said, this website exists, and so I feel a certain compulsion to explain something about PHPAF.
PHPAF is basically a single PHP script containing a small amount of top level code, and the AF class. The AF class basically does two main things. It determines which files should be called and calls them, and it allows classes to be called as needed by simply referencing them like $af->class->method(); without the need to include the class file, or create an instance of the class yourself.
That may not seem like much work to be done, and it isn't. But it does it reliably, and doesn't do a bunch of stuff that you don't ask it to do, which for us is a big thing.
PHPAF has a few class files that are very commonly used, such as the session and respond classes that are so common that they are used by the AF class if they are avaialble. There is also a database class and template class that are used on almost any site that is using PHPAF. But they don't have to be used on a given site, and more importantly don't have to be used in a given situation.
There are other things going on, like automatically checking directories for _prepare.php, _head.php, _foot.php, and, if no file was ever found, checking for _missing.php. But not much else.
Enough for now.