By itself this doesn't seem like a distinctive advantage of Lisp anymore. Lots of languages have REPLs. But I'm told that Lisp is more advanced in some way. Better debugging, breakloops, everything can be modified in-flight, etc.

Interactive interfaces (command line interfaces, etc.) are long available: BASIC, Smalltalk, Prolog, APL, UNIX shells, etc. Some of them also used source level interpreters.

One simple difference is that the READ EVAL PRINT LOOP of Lisp works with code and data in similar ways. It makes it easy to process lists which are data and also to process lists which are programs. READ reads lists, EVAL evaluates lists & other values (and can be rewritten in itself) and PRINT prints lists. Code is lists and programs are lists, too.

In the moment where the REPL uses a source level interpreter, debugging can go very deep, incl. modifying the running source code. That's kind a second nature when one is interactively developing Lisp code: every error stays in the error context and provides a break loop to explore/change/repair the current state - including that each additional error in a break loop just gets us into another break loop level, another level deeper.