You take notes to remember things. So that one day, if you need to, you can look back on those notes and recall what events took place at the time. Everyone knows this, but that does not mean everyone takes care of their notes. One of the most important types of notes are the field notes that police officers take when they pull someone over. And unfortunately, some officers elect to destroy those notes before the trial.
There is no excuse for a police officer having destroyed their field notes, especially if the incident that the notes describe lead to a trial. That is important evidence that cannot be duplicated. If the notes are taken in an objective and fair manner, they can be a clear representation of what actually happened during the incident. These notes are invaluable because they are taken only hours after the incident took place. That kind of event recollection cannot be replicated as well weeks after the fact.
Without these notes, a police officer may claim that he or she cannot remember certain things about the event, and there would be no documentation to remind him. That would lead to a substantial lack of evidence of what really happened for the jury to evaluate. For there to be a verdict that properly reflects what happened, the jury needs to be privy to that kind of information. Destroying notes that could sway the verdict one way or another is irresponsible and should be met with punishment.