Because it is on the shipper's form.
I actually had this come in handy this morning.
We were phoned by FedEx because the driver could not find our house. Turned out the sender had reversed two of the four numbers in the house address.
The phone call meant we got the parcel instead of it being returned to the sender.
If you want, you can use your home phone, your work phone, or your cell phone numbers. Just make sure you can get a message if the call misses you.
