Modular development leads to better programming practices, allows you to share your code, and can reduce dependencies in your final app builds. It gives you peace of mind that when you are building for a specific platform that you will have everything you need, and not have things your do not need.
Let's say there is a new version of a the Cool Plugin on the Unity Asset Store that you really want to try out. If you are using uModules you can pull the Plugin down from Unity, and then duplicate your current project. Add the new Plugin as a module to your project and run
> u pull
A few minutes later you entire project will be ready for you to open in Unity.
Of course, you could also try it in your existing project too, again add the module to your project and run
> u pull CoolPlugin
This will add the new module to your project.
You didn't like the new module. No problem, remove it from your project configuration and run
> u cleanall> u pull
This will rebuild you project from scratch. You did push your main project changes first, right?!?