Not trying to argue, but I thought that the point of PS having declassification ability was so that he could selectively declassify and then leak things. That way, he could not be convicted of the crime of revealing classified info.
But perhaps there are multiple reasons.
That's a workable theory too, but a couple of thoughts to keep in mind.
First, not all leaks of classified information rise to the level of criminal. Only certain select types of classified information carry risk of criminal prosecution on leaking. If the FBI is illegally snooping, and then leaks the substance of that snoop, the leak probably wouldn't be a crime.
Second, leaking, even non-criminal leaking, exposes the leaker to being fired or otherwise disciplined in a workplace way. So, leakers tend to want to do so without being identified. Declassification is not a good tool to get dirt out into the public.
Especially so from the confines of a prosecutor's office, where everything is meant to be kept out of public view to protect the innocent and to protect the investigation. Classified or not, the info a prosecutor is working with is supposed to be the prosecutor's secret.