The relationship between prosecutors and law enforcement is crucial to success. Nothing goes perfectly in a criminal trial; something crazy always happens. Trust is important so that whatever happens can be corrected in the run-up or during the trial itself. While law enforcement and prosecutors sometimes disagree, they are on the same team.
When a relationship is developed between prosecutors and local and federal agents, they feel like they can call on each other for advice. If an agent is on a scene or going to be involved in an arrest and something unusual happens, the agent can call a prosecutor and ask how that would impact the prosecution later. That collaboration is critical on so many levels.