The REPL can be much more than a prompt where you execute code. In fact with many editors you can evaluate blocks of lisp code directly. Networked REPL's are amazing, specially when you're fixing code on machines that are difficult to physically access, like in space. The ability to query the current value of a symbol (function, macro, variable, record) and replace it at runtime without clobbering the runtime state is a great boon to the development process.

Also Common Lisp and a few Schemes allow you to interpret and compile a symbol's value (to speed up execution).