Your reserve capacity is what your heart and lungs use to deal with stress. Injuries or physical trauma, a shocking emotional blow, etc. -- these all demand reserve energy.
Reserve capacity means your heart has the ability to pump more blood, faster in times of stress. Reserve capacity for your lungs allows them to deal with high exertion like lifting, carrying, running or going up stairs. Without reserve capacity, you are much more likely to drop dead from a heart attack or pneumonia when faced with stress.

How's your brain workout so far? In this mind-exercise session, that was your stretching! I know at this point you're saying to yourself, "Are you kidding me -- all those hours of aerobic exercise have been counterproductive for my heart and lungs? Certainly that can't be right!" But it is right. Let me ask you this; have you ever seen a Cheetah jogging? For that matter, have you ever seen any animal jog?
The animals that have the strongest hearts and lungs and the strongest, leanest bodies, are those animals like Cheetahs whose physical exercise consists of; (A) short bursts of highly intense sprinting in pursuit of their prey, and (B) after pouncing on their catch, continue their short-duration-high-intensity-strength-training by using every muscle in their body in a wrestling match. After all that (it usually takes less than ten minutes), they settle down for a big meal.