There's something notable about YouTube's rundown (embedded below) of the top ten "trending" games on its platform last month: four of them aren't even out yet.
Developers have been using beta tests as opportunities to both test and hype their games for years, and this is a notable example of how that totally works -- at least, if you consider YouTube visibility a metric of marketing success.
However, it probably works best if either your studio or the game you're working on is tied to a high-profile franchise. Case in point: Gears of War 4, Doom, Mirror's Edge Catalyst and Overwatch are all games slated to be released later this year that are currently in beta testing, and they're among the top ten fastest-growing games on YouTube last month (in terms of both video watch time and numbers of people making videos about the games.)
We also saw this back in January, when a beta test for Plants vs. Zombies: Garden Warfare 2 jumpstarted interest in the game and drove it to the top of YouTube's "trending" chart for the month.