The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is more than nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable.
That's called a remote debugger, and it's hardly unique to languages supporting a REPL.
I guess it's one of those things that's imho hard to appreciate if you never experienced it.