In the heart, as the action potential reaches the AV node from the SA node, there is a delay of the action potential. This delay is important because :

The electrical signal travels from the sinoatrial node (SAN), which stimulates the atria to contract, to the atrioventricular node (AVN) which slows down conduction of the action potential, from the atria to the ventricles.  This delay in the cardiac pulse is extremely important as it ensures that the atria have ejected their blood into the ventricles first before the ventricles contract. The atrioventricular node further delays impulses by approximately 0.09s. It allows ventricles to receive all the blood from the atria.