The Federal Rules of Evidence dictate the way in which federal and state courts are expected to allow evidence to enter in a case. They determine which types of evidence cannot be used due to being unreliable, and which pieces of evidence should be relied on as primary evidence.
Each court case must follow these rules to the letter; the judge ensures that this happens. In this way, it can be guaranteed that fairness and consistency are brought into each case.
The Federal Rules of Evidence are always applicable and are expected to be known by both parties when taking a court case, especially the plaintiff party. When witnesses are called, their evidence is weighed against this standard to determine whether or not it has any merit.