A layer is just a collection of recipes and/or configuration that can be used on top of Yocto/OE-Core.
The advantage of using an environment such as Yocto/OE (the Poky reference system) comes from the fact that this project handles a lot of metadata (definition files (.conf) of the machine (Raspberry Pi), classes, and recipes (.bb)) that covers everything from simple applications (gpio-packt) to graphics applications such as OpenGLES, EFL, or Qt.
The main motivation of using layers is to organize the long list of providers better and still make sure users may be able to pick only the required or desired provider. It is also a way of providing customizable source code that can be used for any architecture or modified in the way the user needs.
The other advantage is that we can choose all the layers required for each project (from the most basic to the most complex). We can modify them to be consistent with our architecture (ARM for the Raspberry Pi), but a layer can be reused...