I believe it's for those people who need to be present to receive their package.
So If they are not home to sign for the package than it will not be left on their porch. And they'd have to wait another day or two for it to be sent out attempting delivery again.
Meaning they would miss out on receiving their box because they weren't home in time for delivery.
UPS provides the option to pick it up at your own convenience (with in regular store business hours) at a local ups store. (Costs for this service may vary based on locations)
I believe this service would also come in handy to those who track their package,
lets say you preorder the new Taylor Swift cd Reputation, and you have the tracking number memorized, the text message updates and everything counting down the time till your musical package arrives.
What if it's scheduled for delivery at a time you are not home? #Bummer
Taylor Swift is pretty awesome, so you wouldn't want to tempt your neighbors to accidentally borrow it before you got home....
Some good solutions to keeping your prized packages, yours- could be knowing and trusting all your neighbors completely and also installing high tech surveillance systems, around your house, to alert you if your Tswift has been tampered with, while she waits patiently on your porch for your return. Being the captain of a neighborhood watch.
Or, simply have your package delevired to a local ups store, to ultimately ensure that you will be listening to Taylor Swift's album before you neighbors do.
This answer- is subject to modifications changes corrections and deletion. It is only the opinion of a ups employee and is not knowledgeable fact on behalf of UPS company.