Many newer application programmers are realizing the need to open their core functionality to a greater audience. Providing easy, unfettered access to your core API can help get your platform accepted, and allows for mashups and easy integration with other systems.

While other solutions exist, REST is a great way to provide easy access to the logic you've created in your application. It's simple, usually XML-based (we're talking simple XML, nothing like a SOAP envelope), and depends on HTTP headers for direction. Exposing an API via REST in CakePHP is simple.