One of the main advantages of Clojure (and Lisp) is interactive development, the REPL is the base of what can be achieved with interactive programming, it allows you to connect to a running VM running Clojure and execute or modify code on the fly.
There is a story about how NASA was able to debug and correct a bug on a $100 million piece of hardware that was 100 million miles away (http://www.flownet.com/gat/jpl-lisp.html).
We have that same power with Clojure and Leiningen and invoking it is very simple, you just need a single command:
This is what you'll get after running the preceding command:
Let's go into a bit more detail, ...