Or......perhaps it's because they a good place to drop of if you want timely arrival? I do that if I have something critical, I go to the Main PO at 30th Street in Philly.
The Trentonian had a story that they knew where it was picked up, at an apartment complex.
Both Trenton and DC offices are just off main highways. Trenton is near I-195 and the NJ TPK. DC is off Route 50 as it leaves DC.