For me, it is any city/building construction game like City Skylines, Two Point Hospital, Planet Zoo etc.
My spouse and I are gamers. It’s fun to introduce them to a game and watch them experience it for the first time. Every now and then they don’t like a game I’ve introduced, and other times they end falling in love with the game. Right now my spouse is introducing me to Hades, and they’re watching my experiences with it: it’s a good game so far, but they rave about it so I wanted to know what it’s like. We get to hang out while they watch me play, and it’s quality time together. Who knows, maybe this will be a favorite game of mine too?
I don’t watch strangers online play games, but let’s pretend there’s a content creator who I enjoy watching, is close to my age, and I enjoy their personality. How cool would it be to watch that person play some of my favorite games: nostalgic games, ones that inspired me, introduced me to new concepts, broke me, etc. If I aligned with that content creator, I’d love to hear what they think - what they liked and didn’t like about the game, how they felt with the thought-provoking moments, etc.
With that said, some persons like to watch others play games for these reasons and more.
Oh, I get why people like watching games, I’ve watched people play games lots. But mostly I’ve watched when I can’t play a game myself. I had a job with a lot of downtime for a while, and while I couldn’t install games on the computer, I could watch twitch or YouTube all I liked. But given the choice, I’d much rather be playing the games.