I have watched in excess of 100 hours of League of Legends, a game I've played for substantially longer than that. It's an easy, no brainer thing that one can do on an iPhone to relax after a hard day in the coding salt mines, and (unlike the game itself) doesn't require your undivided attention for 40 minutes.
As to what I get out of it: I don't know, similar to what people watching other people play basketball get out of that.
You typically watch the player as much as the game. The game simply provides the context for the entertainment. Also.. Competitive tournaments are a good watch. You may be surprised.
It also makes you better at the game you watch. So watch someone play hearthstone for a few hours and you likely be infinitely better if you are just getting started.
Why are people watching sport? Because it's easier than playing sport, because it's cheaper than playing sport and because those you're watching are better or more entertaining at sport.
