COST: Why UPS might want to use quadcopters.
The most expensive quadcopter on sale at Amazon is $1300, let's assume that it would be three times that expensive in order to have effective range, carrying capacity, and possibly be unmanned (assistance with targeting appropriate landing sites might be required for home delivery). So approximately $4000 a copter. Let's not forget, they'd buy these in bulk, hundreds at a time, they could be significantly cheaper.
I couldn't find an exact cost for UPS trucks, but UPS is slowly changing over to electric hybrid trucks that cost $20,000 more than it's conventional vehicles. On average each vehicle makes 150 stops a day.
If one drone makes a delivery every 30 minutes, then it can make a maximum of 16 deliveries in 8 hours, battery life and charge time could effect this. To deliver 150 packages in 8 hours you'd have to have about 10 drones.
All of this is based on wild estimation, but 10 drones is cheaper than 1 truck. If 1 operator can assist more than 10 drones, they've saved money on Salaries. Cutting down on gas consumption, they've saved thousands.
If the drones deliver to something similar to Amazon Lockers you won't need an operator to assist drones with landings and the drones would need less sensors to help them with flights - most things except collision avoidance and weather correction could be pre-programmed.