Building command line tools in Bash is an extremely tedious and somewhat enigmatic task. There's quite a bit of boilerplate code you're going to have to write if you want your script to do more than just one thing, which will only clutter your script. In addition, your scripts will likely never be able to reference good code you've written from old scripts.
Ash helps you get rid of all of your boilerplate by letting you call functions directly from the command line, while also providing a modular approach to scripting which will allow you to share code between scripts.
You are able to build a module independently that functions as a CLI or as a library (or any combination of the two), and easily share your module with the world.