Why create a separate application for RESTful API?

Question

In the guide for Yii 2 it is said:

While not required, it is recommended that you develop your RESTful APIs as a separate application, different from yo

Answer 1

IMHO if you need REST API for Angular.js or Knockout.js AJAX calls on your website it's an overhead to do it as a separate application. Because you will have issues with cross-domain AJAX calls (especially for POST requests).

I think it's enough to make a module (API) in the frontend for REST API