One problem being addressed is the discrepancy between REPL-based development and non-image-based languages: you have to get your code back out into a source file somehow, without forgetting anything. In image-based languages like Smalltalk and APL, you just save your environment after modifying it interactively; there are no separate source files and the problem does not exist. But in file-based languages, you're stuck using the clipboard to copy code back and forth between the REPL and the source file. It can sometimes be easiest to write the code in the source file, then reevaluate it in the REPL environment. If that process is automatic instead of highlighting specific forms and pressing a button, it can sometimes be a slight productivity boost and less error-prone because you won't forget to reevaluate some change you made in the source.