Aspect ratio.
16:9 (wide-screen) is the aspect ratio of TV, monitor, YouTube videos.
21:9 (cinemascope) is the aspect ratio for ultrawide screen. Some movies with this aspect ratio looks great (the movies where there are black bar on top & bottom are designed for ultrawide screen). So are most games, you get more field of view and a slight advantages from seeing more than others can.
I'm not competitive enough to buy a 25 inch 240hz gsync monitor (and my RTX 2060 can't ever produce that many fps on high/ultra setting anyway). Those are the go to monitors for ultra competitive player who plays in tournaments. I just like a pretty pictures, and more immersion (curve screen) when I play a lot of single player games lol.