The display uses a new backlight made up of thousands of mini LEDs, thus they call it the miniLED display. It's certainly better, especially for HDR content since it can light up or turn off individual dimming zones, thus increasing the contrast and making blacks blacker and bright parts brighter. For HDR content the difference is pretty big, but except for some shows on Apple TV and some showcase youtube videos there isn't much out there you can use HDR with. Netflix does support it if you pay extra, but then it's limited to FullHD resolution whereas Apple TV -at least for the shows I watched, like Ted Lasso- has them in 4k resolution.
To put it differently, I wouldn't purchase a new device without such a miniLED display anymore nowadays, but I would not spend 1k just for that upgrade. There are only a handful of shows that were filmed with HDR support. For proper HDR the cameras used for filming need to support that, and only the latest and more expensive productions do that.
For SDR content the difference isn't big. I use the same kind of miniLED display on my Mac, and SDR content looks about the same. In direct comparison I could make out darker blacks, sure, but that's about it. And even for HDR content when I switch it on and off to see the difference, yes it's there but it's a nice-to-have bonus, not something that's essential to me. And I finished Ted Lasso now, so now I have no HDR filmed shows left to see.