The REPL is really powerful in elixir. You can get a command line into your running app, effectively.

Elixir projects tend to go thru three phases- early its' green code and everything's working, then there's a phase where you find a whole lot of errors and it looks like you're in trouble, but once you get past that you end up with code that you can put in production and literally forget about.

In that second phase, being able to connect to servers and inject data into them and see the results etc can be very useful.