The ability to stock up on staples online
If you're a big fan of Whole Foods' generic brand, 365 Everyday Value, and not going to the grocery store, you'll be pumped to learn that you can already order a whole bunch of 365 items (plus products in the Whole Paws and Whole Catch line) on Amazon, Amazon Fresh, and PrimePantry. That means that as a Prime member you can essentially pick out almost all the staples you need from the comfort of your couch, and have them delivered to your front door.
Allowing customers to order Whole Foods-brand items like cereal, laundry detergent, pasta, and almond milk online is likely also part of Amazon's strategy to introduce the chain to a wide swath of the US mostly unfamiliar with it. As Crain's reports, despite its broad name recognition, Whole Foods really only boasts about 2% of the grocery market in America, a stat that can be attributed to everything from its reputation for being too expensive and upscale, to the fact that the overwhelming majority of its 460 American stores are located in and around major metropolitan areas.
Lots of potential new Whole Foods customers, which Amazon will desperately need to woo if it wants to take over the grocery industry, aren't going to feel compelled to change up their weekly routine of stopping by Kroger or Albertsons unless there is a significant financial incentive and convenience factor to purchase the stuff they want through Amazon instead. That said, the chain is going to have a hell of time trying to win over any new customers who aren't already somewhat interested in purchasing the sort of natural or organic fare that makes up the bulk of Whole Foods' offerings.