By using the combination of the 2, UPS gets Saturday delivery --- cheap, let the Post Office do it, but probably the "we will charge you extra because its a Saturday delivery."
Also I have seen Amazon use this in the past, where DHL or Fed Ex, etc. ships it close to the destination and then the Postal Service takes it from there, like from the major airport to the local PO then out to whereever. And, hey, why not, makes sense to let the people who know the area deliver rather than your deliverer trying to find some obscure place.
Atleast that is the way it was somewhat describe to me awhile back.