It's just an additional sales channel ... if they can have a place where a) people can go to physically preview inventory, and b) can go to immediately obtain merchandise when they can't wait for it.
Additionally, it can help to simplify logistics ... if there are more locations where they person can go pick up their package rather than having to deliver it, surely that can be a boon to their operations.
Now why an ecommerce giant is retrogressing to brick and mortar I can't imagine.