TL;DR: Like the 'M' in JPMS suggests, it let's you compose a system using collaborating chunks of functionality.
It's fair to say that Java modules are designed mainly with the JDK itself in mind. The JDK itself is JPMS' most gung ho user. Modules' main goal in life seems to mostly be to keep riff raff system programmers from poking around in the JDKs internals.
Java modules are a killer idea in my opinion. So it surprises me that it's been adopted by so few in the Java ecosystem beyond the Java platform itself.
I wrote the com.lingocoder.mrjar Gradle plugin [2] as a hobby project. Mostly because I get a kick out of designing and building modular applications.
I'm under no illusions that this plugin will have any impact whatsoever on adoption of Java modules. I'm just putting it out there as a demonstration.
I'm hoping it shows how developing JPMS-based libraries doesn't have to be as onerous as many application programmers at most Java shops seem to think.
[1] https://openjdk.java.net/jeps/261 [2] http://lingocoder.com/mrjar/