When you place some classes together in a Module, you are telling the next developer who looks at your design to think about them together. If your model is telling a story, the Modules are chapters. Domain-Driven Design:Tackling Complexity in the Heart of Software -Eric Evans
A common concern when building an Application following Domain-Driven Design is where to place the code. Specifically if you're using a PHP framework, it's important to know the recommended way to place the code, where to place Infrastructure code, and how the different concepts inside the model should be structured.
In Domain-Driven Design, there's a tactical pattern for this: modules. Nowadays, everyone structures code in modules...