Sometimes it's to experience some of the better parts of the game without having to deal with the parts you dislike. Some games have cool stories, awesome artwork, good movies, etc but gameplay that's too difficult, too much bullshit, etc to bother with. Or maybe people want to see what the fuss is about and can't/won't purchase it for any reason?Serious question, what do you get out of watching someone else play a game that you are never going to play? What's the point? Do you treat it like a spectator sport?
Remember the whole Sekiro thing and discussion of it being too difficult for some people to play(and thus the whole kerfuffle about difficulty modes)? I imagine there's at least a few people who youtubed it so they could enjoy the game despite not having the reflexes/skills to actually play/finish it(and oh, boy, getting any of the remotely good endings requires some major skills to beat the final boss). Either that or use a mod to make the game easier, which no doubt some people will get pissy about too, because FROM games are only for the worthy.
I loved Hollow Knight but after finishing the true ending I was content to youtube someone else doing the Path of Pain and the 42 Boss ironman marathon in Godmaster. Because while I like the game, I also have a pain threshold and the White Palace was enough, thank you very much.
Same with Celeste. Enjoyed it, finished it through Chapter 7, watched someone else play chapter 9 because fuck no I'm not gonna deal with the bullshit of getting all the damn hearts and then the pure madness that is chapter 9.