Why would anyone want to watch someone else play a video game?
I know answer to that question, I know!!
People want to watch someone else play a video game, because they like to watch someone else play a video game.
I believe one of the reasons is that most people 'experience' the game in a different way. It's interesting to see how others do it after you've done it. Sure, you can watch a speedrun as well, but that's a nigh-perfected run done after 100s, sometimes 1000s of attempts. People would usually rather see a run they can familiarize with a bit more.
There are some niche LPs out there that are not watched for that reason, but only for the reason that the viewer doesn't want to beat the game himself, because it's too ridiculous. (COUGHIWBTG)