HDMI 2.1 brings the following benefits, mostly stolen from DisplayPort...
4K120 or 240, 8K60 on a single cable at HDR colour.
It also brings Freesync support, so if you have an Xbox 1X and a TV/display with a compatible HDMI port, you can enjoy frame synchronization.
Also probably why NV dropped the requirement for their G-sync hardware bs and started supporting the VESA standard.