Public: I can think of an example. If the USPS finds out that in a certain area of a certain city, there is a big chance to have riots "tomorrow after 10am" (protests because of X-Y-Z resason), they can alert their local teams to e.g. deliver the post at 7am instead of 11am. Yes, some operations would be impacted (e.g. noon delivery won't happen), but this will protect the staff, protect the items (letters, parcels), the vehicles, etc.

If they just hoard data to feed a bigger best (e.g. NSA) then, the data is still out there (my public blog, your public blog, HN comments, etc.) and they are up for the taking. In which case it doesn't matter if it is a federal agent carrying a NSA or a USPS badge.