The interest of developing on a framework is to obtain a powerful working tool. Today, we must know the reason behind the choice of using a framework or not by a php developer. The goal here is not to do evangelization outright, but rather to give the point of view and clarify the developers who still ask questions about the use of a framework.
The indispensability of frameworks
When you use your libraries you are much more productive. And in general you gain more and more time with each project. Now imagine hundreds of developers pooling all their libraries and you get the main benefit of an open source framework. Have a code tested and managing a maximum of cases that will allow you to code all your sites much faster (once the framework well assimilated).
Open frameworks all use the same structure, the Model View Controller (MVC) framework, which is a proven structure that allows you to separate the presentation part from the logical part. Indeed, this method may seem too complex at first, but it allows you to have a cleaner code and much easier to debug afterwards.
Finally the last asset of a framework is teamwork. Indeed, if you work on several on the same framework is the assurance that everyone code following the same conventions. If your project uses symfony 2 for example, you know that a Symfony 2 dev will be able to work on the code and understand yours from the first day. On the other hand if you use your own framework the adaptation time can be longer, and we take the risk that each new developer makes the structure less and less stable.
Indeed, each framework proposes its vision of things and its way of coding. The use of a framework for creating Web projects is essential and you can not imagine creating an application without using one. The choice to use several frameworks is above all a matter of taste, and it's up to you to see how each framework works and which one you think is most natural.