Look at the next workflow:

1. You create some changes.
2. Write tests (like defsnippet).

Ok. But You not test how you changes working for user jar. For example:

When I tested :on-close option I must test how it worked if I run app from console with command lein run. But for it I must put Quil jar to my local lein repo or update it.

And lein-localrepo lets you do this quickly and easily. And since it's just a plugin, it misses the final assembly. It's just dev tool.