Well, let's consider some options:
1: The FBI was keeping an eye out specifically for Rittenhouse types: anyone that would push back against the Cathedral-endorsed rioters. This is the one we like to jump to, but the late arrival of the footage makes me cautious about that conclusion. If this had been the intent, then the FBI would likely have provided the footage to the prosecution at the earliest opportunity, rather than halfway through the trial a year later.
2: The drones were overwatch for plainclothes agents on the ground. Why the FBI would have people in the crowd is its whole own can of worms, ranging from "agitators" to "partyvan squads hunting specific targets", but if the purpose was direction instead of info gathering, then it could explain the delayed delivery: that wasn't the purpose, so it wasn't until some mid-level puke in the drone department got reminded of the trial that he decided to check the archives and see if they had anything relevant.
3: Situation monitoring. An aerial view of the riots likely gives a better image of the scale of events than cell phone video, and could potentially be used to inform whoever is in charge of advising the President that it's time to tell the governors and mayors to go fuck themselves and send in the National Guard. Hell, maybe Trump even got the chance to witness Kyle in action on exclusive livestream.
4: Mass facial recognition. If we're following the Bezmenov plan, then eventually there will be a culling of the useful idiots, so it behooves the State to identify them now for dealing with later.
5: Coordinating with local law enforcement. The most benign option, but there is the chance that the FBI was using drones to give the cops a real-time read on what was going on behind the lines, giving them the opportunity to head off any advances by the rioters. This would require a pretty in-depth analysis of the police's motions across multiple nights and cities to see if there's any credibility to this, though.
Ultimately, it feels like releasing the footage was a spur-of-the-moment decision. The "analysis" that said Kyle pointed his weapon was done at the prosecution's behest, and we don't know if there was any more footage taken or provided. Like I said, my gut says that this was some mid-level guy hearing about the trial and deciding to check through the archive, and could be either out of a political desire to fuck Kyle OR a genuine desire to facilitate justice. Keep an eye on Ziminski's trial coming up: if we get a second batch of footage, then it's probably the latter.
