- Productivity. Bulding stuff in Clojure is not only joyful but also can give you real (not imaginary, not theoretical, but real, measurable) boost;

- Concise syntax. Less code - less bugs;

- FP. Immutability by default is something that once you get used to it, it is hard to imagine having to write code without it;

- REPL. I mean real REPL, not some kind of crippled "interactive shell". e.g.: Jupyter is interactive shell, Clojure (just like any other Lisp) has a "real" REPL;

Would you use the REPL to inspect / modify / debug running applications in production, or would it mainly be useful for development purposes?

If you do it right, you won't need to inspect/debug prod. But nothing stops you from enabling socket REPL, connect to it and try things there. Just like NASA fixed their failing spacecraft in 1998 during Deep Space 1 mission.