The REPL is the nucleus of interactivity in Lisp.

The first Lisp, called LISP I and its successors, from John McCarthy and his team had a remarkable collection of innovative technology:

* a reader for symbolic expressions. These are nested lists of symbols and other data objects like numbers and strings

* a printer for symbolic expressions.

* an evaluator for symbolic expressions. Each symbolic expression evaluates to a value. Side effects to the running Lisp could be added variables, data or function definitions.

* an automatic garbage collector taking care of freeing no longer used memory

* piece-by-piece allocation of data

* a form of programming with functions, also using recursion

* a way for Lisp to store and restore the main memory heap to/from external storage

* Lisp compiler written in Lisp, which generates assembler, and an assembler which creates code into the runtime -> the first interactive incremental to memory compiler

* a way to program with Lisp code generated by Lisp macros

It had a batch REPL. This Read-Eval-Print-Loop would read a file of expressions, expression by expression, evaluate them and print each return value into a file. It used the above functions READ, EVAL and PRINT.

Lisp was starting up and restoring a default image as the heap. It would then execute the file via the batch REPL and save a new, changed image as the new default image. Next time Lisp starts, it would then restore the latest image.

A next step in the evolution was then to connect a terminal and let the user enter a Lisp expression. Lisp reads it, evaluates it and prints the value back. Side effects again might change the running Lisp. The REPL then waits for the next terminal input. Code could also be loaded from files.

That way the REPL was not some interface which the debugger brings up on request. It was the main interface for the programmer to build up large Lisp systems piece by piece.

We are talking about the early 60s here.

In the coming decades the idea of a REPL morphed into extensive interactive resident development environments like Interlisp <http://interlisp.org>.
Interlisp combined the REPL with managed source code, structure editing for code and data, automatic error correction of code, undo of evaluation effects, a virtual machine, ... and a lot more. Here Lisp, the IDE and the programs under development were running in one Lisp runtime. We are talking about the early 70s.

Interlisp moved at Xerox PARC onto their new graphical workstations. Thus the REPL-based development environment moved to graphical user interfaces. The Interlisp team got an ACM Award for their pioneering work.

The MIT AI Lab took that idea to their graphical Lisp Machines.

Today in 2021 Lisp programmers often use SBCL as the implementation, GNU Emacs and SLIME (or similar) as their development environment. It's still usual to develop a running program, having potential multiple REPLs into the program, using Emacs/SLIME to send code for remote evaluation to the running Lisp.

GNU Emacs itself is a Lisp program, too - with its own integrated Lisp and the development tools for it. GNU Emacs has its own REPL, called IELM, the interaction mode for Emacs Lisp.

Even in Lisp not all development interactions goes through a REPL, but there are still a few very powerful ones, like those of Allegro CL, LispWorks, McCLIM, Interlisp (recently open sourced) or the Lisp Listener from Symbolics Genera. Typical is the rich interactive error handling in multilevel break loops with restarts and the ability to continue from errors. The above also support image-based development and integrate Lisp, IDE and the software under development in one program.