Why is there a non-conducting layer between the atria and ventricles?
The atriae need to contract alternatively to the ventricles, which is why it is necessary to isolate the two in terms of conduction.
The atriae and ventricles need to contract separately so that the heart can maintain the pressure needed to work as a pump. If there was continuous conduction between the two chambers, the atriae and ventricles would contract at the same time and blood wouldn't be able to circulate. With the non-conducting layer in place, the atriae are able to work whilst the ventricles rest, and vice versa.
Listen to your heart beat, you should hear two separate sounds. The first is your atriae contracting, the second is your ventricles contracting - before it starts all over again, at an average rate of 70 times per minute