When two or more brands come together in order execute a marketing strategy or campaign, magic happens. All of a sudden you are enhancing each other's reputations, you are gaining audiences in an organic way, you both have new unique content, your brands are exploring previously unknown places.
The more partners you bring into a campaign the better, faster, stronger your momentum becomes. You go from only reaching your social audience to reaching all the other brands' audiences and beyond. The only other way to get a captive audience of that volume would be to shell out big bucks in advertising, and let's face it, free is always better.