It's a stealth satellite, Northrop Grumman's version of NROL-21, which was probably a lot like MISTY. Lockheed got to do two, Boeing got to do one (and it fucked up and got shot down), now it's Northrop's turn.
It's perfectly positioned for observation of embargoed shipping. It was observed venting correctly, in orbit, over Sudan.
A (Radar) satellite in this kind of orbit therefore would be very useful to keep track of illicit shipping movements on the High Seas.
Think stuff like embargo-runners, e.g. embargo-breaking shipments of coal and oil to for example North Korea, illegal weapons exports from North Korea, oil exports from Syria, illicit weapons transports to the Middle East, and human trafficking as well as drugs shipments.
Ships engaged in such illegal activities sometimes turn off their transponder, making it harder to track their whereabouts once out of sight of landbased shipping radar (see also the story about one particular embargo-breaking ship here). The classified US NOSS duo ELINT satellites and similar Chinese Yaogan triplets are meant to track ships from passive radiosignal crosslocation, but when a ship displays strict radio silence, these systems will not detect them either. But Space-Based Radar will.
Embargoes have become an important geopolitical tool when outright war is deemed not an alternative. We currently see embargoes enforced with regard to for example Syria and North Korea. Means to enforce embargoes including detecting and stopping potential embargo violations therefore have become important. Human trafficking and drugs trafficking are growing geopolitical problems as well.
So was Zuma meant to be an (experimental, i.e. a technology demonstrator) version of such a Space Based Radar for Ocean shipping surveillance? It is an option.
What might argue against it is the extreme secrecy surrounding the launch. Very few details were made public about the Zuma payload, the Agency operating it was not disclosed, and the launch was announced very late.
"I would have to direct you to SpaceX, who conducted the launch."
That's a Pentagon official. A DoD official. "I would have to." If it were a DoD launch, she wouldn't "have to" do anything. She could choose to. If it's a CIA launch? Well, then she has no legal standing to discuss it. More than that, it isn't her budget so she doesn't have to say shit about it.
You know, thinking about it,
That's a couple pioneers looking through a telescope. If the goal was to check the observability and stealth characteristics of a new mockup satellite by getting it within 12,000m of the ISS, that'd be a damn good patch to go with.