I was expecting a comment on sports, but the difference is that most people cannot play sports* because they don't have the fitness to last longer than a few minutes.
*at least those sports who are normally broadcast
It's ok to watch an "expert" like a cook, an artist or an artisan at work.
But when talking about games it's quite different. Games are created to entertain people who play. In the past there have been some crossing into entertaining those who watch, for example chess and poker (mostly watched by other players who wanted to learn to get better), but tabletops, RPG and videogames hadn't crossed that line yet. Maybe some are watching them too to learn to get better, but this is already a bit controversial: getting better to what purpose? If you plan on competing then there's a purpose (even win money). And maybe that could really be the point why it worries me, that people are looking to turn simple genuine fun into yet another competition, as if we didn't already have enough of that. I guess there is still sleep to be non-competitive, so far at least. And the second reason for being disturbing is that it kind of feels like a huge "giving up" when you are spending more time watching someone else doing something fun than doing it yourself.
I like to watch people play games for a variety of reasons.
One, to learn from them. For instance, I have almost a thousand hours on EU IV. I thought I was pretty decent at it. Then I started watching people like DDRJake (before he went to Paradox), Arumba, and Florryworry and quickly realized how much I could learn from them. In the Civ series, I watched MadDjinn to get better. When I raided hardcore in WoW, I watched the highest end players of my class to see how they reacted to certain situations.
Two, I enjoy their presentation of the game. If I found the host engaging, I'm more likely to watch them. I've cut out most of my TV watching (pretty much only the news and sports are left) and have replaced it with Twitch/Youtube watching. Generally speaking, I'm happier. Although I do have to catch up on Rebels.
Three, and this is specifically about RPGs, I enjoy watching others take part in hobbies that I also enjoy. I like to see what they do and imagine how I would run such a scenario or if there is a NPC or quest item I could steal to make my games better. Most of the time I have these games on at work like a podcast. I've listened to hundreds (probably closer to a thousands) of hours of gameplay. I've learned things that make me a better DM and a better player. I'd say it's worth it.
You don't have to watch and I respect anyone who doesn't or doesn't have the time to watch. But I really like Twitch and recommend it to everyone to at least try and see if it would be something they like.