Amazon agreed to pay just shy of $1 billion for video game streaming site Twitch last night. The service, which originally launched as a spinoff from in 2011, has become so wildly popular that its owner’s major concern has not been how to grow the site, but how to secure sufficient servers to supply the sheer number of streams its users both create and demand. It’s obvious to everyone that millions across the world want to watch other people play video games — more than 55 million people use the service every month. Less obvious is why.

There’s no easy answer, but the most popular streamers offer viewers one or more of three key benefits: buying advice, a community built around a streamer's personality, or the chance to observe professional grade skill. These elements, coupled with Twitch’s ease of access and gamer-friendly attitude, have contributed to both the company’s billion dollar valuation, and a huge change in the way millions of people play and think about video games.