Take THAT, Dota 2!
Valve's free-to-play, high-skill-ceiling MOBA Dota 2 has set a new record for most concurrent players on Steam, with an impressive 329,977 players at once. The last game to set the record was something called "Dota 2," with a less impressive 297,000. This influx of Dota-ers is likely due to the qualifying matches leading up to 2013's The International, aka the biggest tournament in eSports.
Watching tournament games in the client itself has its own benefits, leading some viewers, myself included, to prefer to watch games that way. For example, during the first kill (first blood), multi-kills, and victories, a single item is given to a single viewer within the client. That could be me! Sure, there are literally 60,000+ other people usually watching with me, but a man can dream. Though, I think I have better chances of actually being in the tournament.
Watching games in-client will also likely going towards leveling up the courier gained from purchasing the Interactive Compendium. How it will evolve is not known just yet, but considering it has a running tally of your watched games, it's safe to say that has something to do with it. There's also the Queen of Pain aesthetic set, which can also evolve by watching games from the Premier League. I bet a large portion of these concurrent players were watching games instead of playing them!