If you think of cables as pipes, you need a bigger pipe to transmit a 4K signal than a 1080p one. A 3,820x2,160-pixel 4K Ultra HD signal sent over HDMI is roughly four times the amount of data as an HD 1,920x1,080 signal. When you increase the resolution of a TV signal, the amount of data of that signal goes up. This is the same as HDMI 2.0a/b, and is what makes wide color gamut possible. On the color front, 2.1 supports BT.2020 and 16 bits per color. You should be able to get 4K/60, and a basic 8K/30, with current cables, but the rest will need an Ultra High Speed HDMI cable. The increased resolution and frame rate possibilities are a futurist's dream:
For most people it's not a good enough reason, on its own, to buy a new TV right now. That doesn't mean you need HDMI 2.1, however. In fact, to get the most out of those consoles you'll need a TV that supports at least some HDMI 2.1 features. HDMI 2.1 is also on both of the next-generation game consoles, the PlayStation 5 and Xbox Series X. It's currently found on the best new TVs, including recent models from LG, Samsung, Sony, TCL, Vizio and more. The latest version HDMI is called 2.1, and it's a big leap forward compared to HDMI 2.0.