kind of a catch all, if you have a g/freesync compatible monitor it'll ensure you're at least not creating a larger frame buffer than necessary which introduces the crazy input lag. If you use Nvidia fast sync instead of regular/default vsync it might introduce some weird microstuttering if the fps is not capped to the rounded refresh rate.
RTSS + fast vsync just ensures you get the absolute most out of the technique, microstuttering is fixed with an in-game frame cap. Source engine and lithtech are the ones I've ran into so far where you needed said in-game cap.
Why the does said buffer cause stuttering? No fuckin clue