We have so many alveoli because it increases the surface area in the lungs for gaseous exchange. We need a lot of oxygen in order to respire at an efficient enough metabolic rate.
Not only does that mean we take in a decent amount of oxygen needed. But it also means we can transfer carbon dioxide out of the body at an efficient enough rate so that it doesn't build up in our blood.
Just think about how much more oxygen you take in when you breathe properly.
It's important that we have enough surfaces of exchange to allow sufficient amounts of oxygen in and carbon dioxide out or we'd all be out of breath constantly as well as have weak muscles due to oxygen deprivation.
Gas exchange of oxygen and carbon dioxide takes place in the alveoli. Oxygen from the inhaled air diffuses through the walls of the alveoli and adjacent capillaries into the red blood cells. The oxygen is then carried by the blood to the body tissues.
Bronchioles begin with rings and then slivers of cartilage which helps to keep them open. As they become smaller and smaller, the cartilage is replaced by smooth muscle. These smooth muscles regulate how much air comes into the lungs.
If the sympathetic nervous system stimulates cardiac muscles to increase the heart rate, this causes dilation of the bronchioles of the lungs (increasing oxygen intake) by relaxing the smooth muscle.
Stretch receptors in the walls of bronchi and bronchioles are activated when the lungs expand to their physical limit. These receptors signal the respiratory center to discontinue stimulation of the inspiratory muscles, allowing expiration to begin as the muscles relax and return to their original size. This response is called the inflation (Hering-Breuer) reflex.
Problems with relaxation of the smooth muscles can cause problems associated with emphysema and asthma.