It enables you to use $names in places where you would normally use values. I typically find myself changing values in many locations in my css, and it gets really tiresome if I have multiple files.
All this being said, SASS or any variant like that is a better solution, but this is a quick no pipeline solution for small projects and prototyping.
Read more here: https://github.com/AlexanderBrevig/jcss
Here is a simple example index.html that uses inline styling and multi-file load of .jcss files: