Activision hasn't exactly downplayed what its expectations for Bungie's Destiny are. Just yesterday during an earnings call, CEO Bobby Kotick proclaimed that he expects it to be "the best selling videogame in history." Ambitious, to say the least.
One of the ways that Activision plans to build hype for Destiny is to launch a beta. During the same earnings call, Activision Publishing president Eric Hirshberg revealed that the company intends to hold this beta event sometime in the summer. To move copies ahead of time, the beta will be made available to those that pre-order the game.
A large-scale public beta sounds like a pretty standard move considering what Activision's looking to accomplish. Apart from the whole "best selling game ever" thing, it expects Destiny to be its third billion dollar franchise. Getting people talking about it as much as possible isn't exactly a bad marketing strategy.