The REPL is part of my TDD loop... Which is actually, write a test, come up with a plan to make it pass, google for the programming constructs I need in the language I'm using, test a snippet (or a fully functional line) in the REPL, copy over to my project, make the test pass, ..(refactor). Without the REPL I would have an extra cognitive overhead of wondering if I had introduced a bug through misunderstanding a sketchy part of the docs.