Craig: they're not trying to attract anyone. In-store pickup, curbside pickup, ship-to-store, returns, etc. are available at every Macy's store. The only thing unique about the Dover and Littleton stores is that you can't shop in the store. Presumably in-store sales have been weak in these locations anyway. By turning those two locations into "dark" stores, Macy's can make them more efficient for fulfillment and reduce shrink. I'd expect the Dover store to fulfill online orders for much of the Delmarva peninsula and perhaps some of the Philly metro area.
The stores in the Philly market probably do much higher sales volumes. (With the possible exception of Exton Square, but that store is literally in the middle of the mall: no direct parking lot access!) Also, I believe the Dover location is a single story, which is unusual for Macy's stores and helpful logistically if you're turning the space into a fulfillment center.
But Macy's mall real estate is not high priced. The Dover store, for example, is ground leased. Macy's probably pays no more than $4/foot including utilities, taxes, and common area charges: significantly less than it would pay for industrial space. There's a reason why Amazon is interested in taking over vacant mall anchors for distribution space: the inefficiency of not having a purpose-built space is at least partially offset by extremely low occupancy costs.