Why would I want to watch someone else play a video game when I could... I don't know play the game myself ?
Many reasons.
You may be a new player wanting to learn a game, or some of its mechanics/contents, and it's a good way to do so since you can discuss with the streamer about what they're doing.
You may simply enjoy someone's personality.
You may really enjoy someone
personality.
The only legit one: tournaments.
As a general rule, plebs (and that's precisely your thought) will only watch a stream, and that's it. They will spend X hours of their day watching a stream, maybe participating in the talk, writing wall of texts that nobody will read in the tiny chat, asking questions no one will ever answer because said questions are HELP I AM TRAPPED IN 2006 PLEASE SEND A TIME MACHINE, and so on. This is the usual stream viewer, or stream monster as they're called in some communities. Those people are more or less a waste of data but are pretty useful to pay 4,99$ for a subscription.
The advanced viewer will have two screens and will play/watch a movie/work/whatever as they listen to the stream, or watch it randomly. These people understand what a stream is for and have some kind of maturity toward it.
And then, you have the person who understands the true value of streams and doesn't need any more than that: this guy watches tournaments only, whichever they are. That being said, if we're talking LoL for example, Youtube will provide a better quality.
There are many reasons to watch streams but at the end of the day, the reason is very personal and is more often than not objectively HELP I AM TRAPPED IN 2006 PLEASE SEND A TIME MACHINE, but not for the reason you'd think.
Why would I want to watch someone else play a video game when I could... I don't know play the game myself ?
and good streaming commentary is a nice bonus).
That's actually one of the bad points.
Outside of very specific fighting games, most if not all of the commentaries offered are down right garbage for 2 reasons: fake hype that nobody care about and simply Bronze level of game analysis.
There is a huge difference between a Yipes, a James Chen, a LoL shoutcaster and whatever ***I have heard when I watched various Smash streams.
And whenever you dive into anything that is way below "professional" standards, outside of the 2 our of 10 guys who are really clean and trying to offer good commentary, all you will get is HELP I AM TRAPPED IN 2006 PLEASE SEND A TIME MACHINE nonsense.