Let's guess for a moment this ISN'T anything more nefarious then spying. I.E. no nuclear, chemical, or biological payloads.
Data they may be after
1.) Targeting. Satellites especially theirs's are more limited. Their supersonic nuclear missiles are only as accurate as their data. So for a cruise missile they may want more accurate data for targeting. It made a B line for our ICBM fields. A very likely target in a first strike scenario. It could even be used for targeting in an immediate strike, but if not the data it records could be very useful for such sorts of weapon systems, cruise missiles etc. It also could be for an EMP plan.
2) Maybe there has been a substantial change they are concerned about. We agreed to several treaties for nuclear arms reduction, it is unlikely they and the Russians have been faithful unless for financial reasons, perhaps they have reasons to suspect we have not been faithful on our end either, and want a closer look. Perhaps this is even the reason we are allowing it to pass over. A closer view then satellites might make it more clear the readiness of silos, and or building of new ones. Or refitting of older ones that were supposedly mothballed etc.
3) They want to get a better idea on pattern for a future biological or chemical attack. Their own generals since the 1980's have told their own military that an attack on the United States would likely be biological. With early attacks something with plausible deniability (not unlike covid) and then a later more serious more fatal pandemic, and then last a direct release of anthrax etc when our defenses are down, They very much do not want to use Nukes, as they hope to seize the American breadbasket for themselves. This is no joke there are numerous videos and translations of their brass discussing this. Even if this isn't the attack per say, it might be a proof of concept or trial run.
Everyone who wants to remotely be informed about China, and the threat it poses, and has seriously planned for the U.S. should read this. This is after all their own words where they outline their war plan for us. .
The Secret Speech of General Chi Haotian
Also not unrelated.
'We're going to lose fast': U.S. Air Force held a war game that started with a Chinese biological attack