So for me, it depends on the type of game and the content creator. When friends have asked me about streamer culture, I tell them to jump around and find a creator who thinks/makes decisions loosely the same as them but more importantly has a generally positive vibe in both themselves and their community - not everyone is sugary sweet screechy nightmare.
For story-based games, I get to experience all the artistic aspects (story, acting, level design, animation etc) without having to learn and/or put up with crummy controls, be distracted by poor performance or optimisation, get caught in thankless grind mechanics, or simply get experience a game in a genre/on a platform I wouldn't ordinarily.
Watching someone who is good at creating narratives play an adventure-crafter/ management sim adds lots of additional layers of interest, where I would otherwise check out after a couple of sessions playing by myself.
Shooters are less of a draw for me, maybe because I still actively play them. I have never clicked with any individual creator but find groups quite compelling, especially when the game has more tactics/strategy involved and they need to work together to succeed.
I haven't thought much about it but even as a child, when in a group I much preferred watching someone play than having a go myself and I guess that followed through to YT/Twitch when they came online.
metalingus