Dive Brief:

• Of all the perks that Amazon members enjoy — streaming television and musical entertainment, an e-book lending library, photo storage, among others—the real reason Prime members shell out their $99 yearly dues is the free, no minimum, two-day shipping on many items, according to a study by RBC Capital Markets.

• In fact, the percentage of Amazon Prime members who say they prize the free shipping option the most has crept up to 78% this year from 74% in 2013, far outpacing other perks.

• Fewer than one in 10 Prime members signed up because of streaming video, for example, and a paltry 1% joined because of its Kindle Lending Library.

Dive Insight:

That more than three quarters of Amazon Prime members know exactly why they’re there is an just another impressive stat among many about the e-retail giant’s membership program. (Another, of course, is the program’s massive conversion performance.) 

Amazon has been working hard to add more items that qualify for free two-day shipping, but it’s perhaps hard to fathom why it’s also working to add other perks to the membership.

One speculation: Perhaps Prime members come for the free shipping, and stay for the streaming.

In any case they are staying: Investment firm Macquarie estimates that 40 million U.S. households are Prime members, and current growth rates could bring Prime to half of all households by 2020.