A uniform API for accessing stuff from different kinds of databases, protocols, and 3rd party APIs.
Use the same syntax for all of your models. That means you write the same code to get and store your business objects, whether its data source live in MySQL, MongoDB etc.
It gives you the opportunity to clearly implement validation directly at one point in model layer before you save or update records.
Declare and manage your model relations regardless of its data storage.
There are some built-in queries, adapters and framework integrations, but if you need something more, just write your own extension based on official API.
You don't need any framework, extension or third-part library. Everything you need is PHP 5.4+ and some nice application for the use of this tool.
Everything is licensed under the MIT license so you can use it in your applications arbitrarily.