This new Windows 10 feature is intended to augment NVIDIA G-SYNC and AMD FreeSync:
This new OS support is only to augment these experiences and does not replace them. You should continue to use G-SYNC / Adaptive-Sync normally. This toggle doesn’t override any of the settings you’ve already configured in the G-SYNC or Adaptive-Sync control panels.The VRR option is exclusively available on PCs with a GPU with WDDM 2.6 or above drivers, that supports G-SYNC / Adaptive-Sync and this new OS feature, and a display with Adaptive-Sync or G-SYNC support.
This new toggle enables VRR support for DX11 full-screen games that did not support VRR natively, so these games can now benefit from your VRR hardware.