Your products, not a copy of them
Add a product once and it appears in both places. Change a price, a photo or the stock and the app follows, because it is looking at the same catalogue rather than a synchronised duplicate.
Orders land in the same place
An order placed in the app arrives in the same list as an order placed on the web, with the same statuses and the same notifications. Your fulfilment routine does not change.
What a phone can do that a page cannot
Push notifications to bring people back, camera and location where they help, and a home-screen icon that does not depend on someone remembering your address.
Published under your name
The app carries your brand, your icon and your colours, and goes to the stores under your account, so what you build stays yours.