http://www.cracked.com/personal-experiences-1908-were-all-speed-6-secret-realities-pro-video-gaming.html
Whenever there's an article about competitive gaming people are coming out of the woodwork to complain about how it's not real sports and how can you enjoy watching other people play videogames(?) and I find myself reflexively defending it.
1 - It's real sports. The top players are practicing many, many hours every day while constantly playing in tournaments. You're expected to be in great physical and mental shape AND in MOBA's (DOTA 2, Heroes of the Storm) you also need a modicum of social capabilities since you're playing in a team. Puppey, former team-leader of the then worlds best DOTA 2-team Navi is known for a being a really social and easygoing person. Starcraft 2 on the other hand has a lot of man-children among its top players sadly but there's exceptions like HuK and WhiteRa. Starcraft 2 even has its own sports injury in carpal tunnel. They move their hands so fucking fast their wrists start breaking down (insert masturbation joke)!
2 - Being a fan of professional DOTA 2 is the same thing as being a fan of professional soccer. You have your favourite teams and players that you root for, there's certain tactics that you dislike (I hate Russian DOTA :P) and you know an absolute shit-ton about the game. I'm not an emotional guy and the screaming when a team makes a goal is not something I will ever do but I still fist-pump when my team wins, which is the closest I'll ever come. Thanks to DOTA 2 and Starcraft 2 I finally understand what people enjoy about sports, before I thought the whole thing was retarded.
3 - There's even professional commentators i.e people who pay their rent by only talking about the game during tournaments and similar. They're often players past their prime whose knowledge of the game is still world-class but maybe they can't spend 10 hours a day just practicing. They're also often really funny with quite a few comedians in the ranks. Husky was a Starcraft 2 commentator who was huge for a while, mainly for his infectious enthusiasm. TobiWan of Dota 2 has a similar style where he speeds up his talking whenever there's a lot going on because he thinks this is so fucking awesome guys did you just see what that guy did!?
As for Let's Players on Twitch and similar, like the article said they're often funny and/or likable and will interact with their fans, do things for tips or just for the hell of it. I follow Chu8, a minor celebrity in his own sub-culture who seems like a great guy in general and who's absurdly good at MOBAs. Twitch always has a comment field next to the stream where people are hanging out and talking about the players performance, asking questions and making lots of stupid jokes.
People are watching other people play games and enjoying it for various reasons, what is so hard to understand!?