I'm sure they could still save money because it could be less time sensitive and would include only one doorstep delivery for all orders instead of one per order.
One order may need to go by air if it's at a distant warehouse, but another may go ground because it's available close by.
I'd like it solely for the convenience--several times I have wanted to order something but knew I would not be in town when the delivery happened. I'd love to say "deliver it to me next Wednesday" and Amazon figures out the cheapest way that would work instead of having to remember to make my Prime order on Monday.
