Increased productivity: With a single codebase, developers can quickly identify and address any issues they encounter. This makes the development process more efficient and helps the team to get more done in less time.

Ease of creating new modules, with no need to set up infrastructure, continuous integration/continuous delivery, databases, etc.

The simplicity of debugging.

Ability to easily split the application into different services and convert the project to microservice architecture.

Improved team collaboration, as responsibilities, can be divided across modules, and merge conflicts are minimized, compared to traditional monolithic applications.

https://medium.com/javarevisited/exploring-the-benefits-and-challenges-of-a-modular-monolith-architecture-5ee9f69988c8