A REPL does a good job of proving your code works as intended now. If you save that code in a file and run it regularly (which is a fine way to start making a test suite), then you can prove your code still works later.

That's especially important when multiple people work on a project: you have some chance of remembering what important REPL incantations prove a system works. They won't. If you haven't put your REPL magic in a test suite, then they're going to break your stuff.