The reason a phone number is required, is due to the delivery service RCm8 use. As part of the service, a text is sent notifying you of when the delivery is due arrive.
As the parcel gets closer, you are notified to within a 15 minute window. You can even track the driver down to the minute.
This is a great service and even gives you the option to change the delivery date or contact the driver with additonal notes regarding your delivery.
The above service makes the deliveries very successful. Rather that than waiting around all day for your parcel :)
This applies to next workding day deliveries.