In essence it's to further the business goals of both organizations involved. Either to increase brand awareness, or sell more product. The reasoning behind the Starbucks and Barnes and Noble Joint Venture was probably the understanding that coffee keeps customers in book stores longer, making them more likely to stay. Someone heading into a book store for a coffee, might then be tempted by a book purchases. More Starbucks in other stores, the more physical locations they occupy, increasing their potential market share.