Naturally I don't know how the Flock search platform operates but this implies to me that this "Reason:" field is a mere formality, after-the-fact accountability (if even that!).

If the search was granted based on human checks for the reason given, they would not dare be flippant about this. Of course it would take more time, humans don't scale, etc. but that's kinda the point: invasive methods should be a last resort.