REPLs are for interactive development, where you try out your code live on a command line, insert working snippets into an editor, and gradually build up a working program.
Really shines when the language / REPL has decent introspection, is able to break out into the REPL prompt in the middle of your code with in-scope identifier lookups, and you're dealing with lots of libraries that you don't necessarily have encyclopedic knowledge of, yet need to get the job done under time pressure.
At least that's how I use REPLs, most usually, pry in Ruby.
Really shines when the language / REPL has decent introspection, is able to break out into the REPL prompt in the middle of your code with in-scope identifier lookups, and you're dealing with lots of libraries that you don't necessarily have encyclopedic knowledge of, yet need to get the job done under time pressure.
At least that's how I use REPLs, most usually, pry in Ruby.