Originally Posted by Imouto
It is used together with the VRR solution of your choice for those times when the FPS is above the max refresh of your monitor.
In that case, it's never a good solution for high refresh rate displays. I've tested Fastsync in all possible scenarios, and FPS capped V-sync VRR is much better.
100+ FPS 60 Hz non-VRR Fastsync was pretty good IME.