To my experience, for small apps or very small services you are right, maven modules make thing unnecessary complex in two ways: technical and cognitive.

However, modules can be of much help when:

- Services are complex and we need a way to visualize their most relevant boundaries so that developers can find or add features in the right place quickly.

- Decomposing services. Even microservices can be composed of several but smaller services. Using maven modules can provide us with valuable feedback about the interaction of the components and the best way to decouple them. Or a reason to leave where they are. Moving code to a standalone process will be easier this way.

A well-modular application can enforce a healthy separation of concerns too and reduce the coupling between elements of the application. 

Modules can be the tactical answer to a strategic question or need too. For example:

- We need/want to develop, evolve and maintain modules separatelly, providing them with different SDLC. 
- Versioning concrete pieces of code or logic
- Code reuse (as shared libs)
- Archetyping (look for maven archetypes for more information)

If the application implements a hexagonal (micro) architecture, one more advantage could be on the "adapters". These can be implemented later and replaced without changing the core of the application, or the logic of the module we are working on.

Nevertheless, whether these things are advantages or not depends on the context. You have to decide if they are relevant or not. 

Worth nothing to say, sometimes developers use these and other resources just because they can or because they became their golden hammer, regardless of its suitability.