Besides being a much larger target a double lung shot will put down an animal much quicker than a heart shot. This is with arrows and not bullets.

When you double lung an animal the blood pressure instantly drops and the heart instantly begins beating faster to try and make up for this drop in pressure. The faster it pumps the quicker it bleeds out and the quicker the brain and muscles are deprived of oxygen.

On a heart shot the heart can't pump blood with a hole through it and the blood just drains out rather than being forced out. It takes longer for the brain and muscles to be deprived of oxygen allowing the animal to cover a larger distance.