As you guessed, the history tracking is one of the killer features. Imagine it being super easy to edit the history of a REPL session (delete, reorder, merge, and edit contents of each command) and rerun... That's a notebook! Notebooks also allow for markdown input and rich HTML output (which is killer for plotting) making it possible to polish your REPL history into a document you'd actually want to share with a colleague to explain something like a data analysis workflow.

I actually started in notebooks and then learned to love the REPL as a simplified "scratchpad notebook." I'd say in many ways notebooks are an improvement that cater heavily to REPL-lovers, but that for some quick tasks, the extra complexity isn't always worth it.