With lisp, I edit, eval (simple command in my editor), see result. If I'm building a game or a long-running app, I can redefine functions while it's running without doing a full recompile and having to try and get back to the state the app was in when I wanted to test my change.

So you get the ability to live-code while the app is running. Once you get used to being able to do this, leaving your editor and opening to shell to test your app seems like a barbaric exercise.

So a good REPL (lisp or not, doesn't matter) can make the experience a lot more interactive and experimental that your average write -> save -> compile -> run cycle.