This happens to squeeze out blood completely, else the heart may not function properly
The electrical impulses are carried by bundle of His and send to the bottom of the heart via purkyne fibers. This happens in order to spread the impulse from the atrio-ventricular node to the sides of the ventricles. Now in the next round of impulse causes the ventricles to contract and squeeze out all the blood from the ventricles. If this does not happen then the heart will not be able to perform to its full capacity. Atria may also not release its entire blood into the ventricles.