In this context, 'render' means transforming React (or similar framework) output into a DOM tree, plus the magic that goes into rehydrating dynamic elements once the static copy of the page is loaded.
The benefit of that is getting all the effects of 'just plain HTML' while still being able to dynamically update parts of the page, and doing the entire thing in a single framework such that (if you do it right) your code doesn't have to explicitly do anything to specially handle the two different use cases.
