OCaml has a REPL. I use it all the time to check that a new function I just wrote is correct. Yet I still get huge benefits from the static typing: many of my errors are stupid type errors, and having the type checker tell me about them, rather than a runtime error (or worse, a wrong result), makes early prototyping much faster.

Even if I already have a REPL. I believe the main reason is because the type checker is much closer to the source of my errors than runtime checks or tests are.