It is too early for Amazon to declare victory for its 3rd annual Prime Day, but the signals look good. Revenues from the highly discounted sales during the 30-hour event are likely to top $1 billion for the first time. A big chunk of the sales will involve Amazon’s own products, like the Echo, Dot, and Kindle. And that is the real prize for the company. While Black Friday is about volumes, Prime Day is about platforms.

Amazon already controls the largest U.S. online marketplace, accounting for about 40% of all online retail transactions. However, in order to maintain the 20% annual revenue growth it has promised the markets, it needs to add about $28 billion in new revenues each year. Amazon Prime is a great way to get there.

Prime subscriber numbers are hard to come by, but most sources estimate that there are about 80 million Prime members worldwide, most of them in the U.S. Each member pays about $100 per year for free two-day shipping and other benefits, like preferential access to Amazon’s content libraries. Thus, Prime accounts for about $8 billion a year of direct revenues to Amazon (amzn, -0.36%). However, that is just the beginning. Prime members spend about twice as much annually as non-members on Amazon sites—about $1,200 compared to $500. Thus, Prime members account for a massive $96 billion of Amazon’s $135 billion in annual revenue.

Prime Day deals are only available to Amazon Prime members. For this reason, Prime Day has always been more about driving new Prime memberships than new sales. By this yard stick, the annual event has been phenomenally successful. Even though Prime has been around…