Faster and cheaper
The taxis are being guided through the Flywheel app, which sends them to distribution centres, which in turn may hand out up to 10 packages. These then get driven to the customers in question.
Amazon believes there is a future for this means of transportation as they are faster and cheaper than couriers. They can also keep transporting people, although that might be trickier to organise than the theory suggests. The most difficult part will be to calculate which taxi should take the package(s) and in which order.