Great question. These preservation letters could be important. FBI sent the letters. If the information was not retained by the requested parties then they have obstructed an investigation. There would also be official misconduct. This would be for each department sent the letter(s). Could be very big. Will show conspiracy between the Agrncies involved.
if the records were destroyed, obstruction would seem to be easier to prove than the underlying crime, while the underlying crime is much worse than the cover-up in this case