Good question about the value of a REPL to Python. My first experience with a REPL was the SQL*Plus tool from Oracle. I don't remember it being called a REPL. But, as I read about Common Lisp's REPL, I realized that I had been using one.

Early in my experience with Oracle's DBMS, I found the REPL handy to work out ideas. But a REPL is essentially a single line editor from which you can evaluate expressions/statements. As my ideas grew, the REPL became more of a hindrance. Having a full screen editor became more necessary.

Nowadays I work out all ideas within an IDE, firing up the REPL only for quick data checks. So my answer is that a REPL is helpful but not necessary.