First of all, what is temperance?

The dictionary defines temperance like so:

moderation in action, thought or feeling; restraint

Temperance is important to Health because not only is it unhealthy to have a lot of junk food, it is unhealthy to have an excess of food that is good for you as well. Exercise in excessive amounts can be bad for you. Excessive water can be bad for you. Too much sun is damaging as well. Everything we do NEEDS to be in moderation.

Sr White writes in the Testimonies: 

Intemperance of any kind benumbs the perceptive organs and so weakens the brain-nerve power that eternal things are not appreciated, but placed upon a level with the common. The higher powers of the mind, designed for elevated purposes, are brought into slavery to the baser passions. If our physical habits are not right, our mental and moral powers cannot be strong; for great sympathy exists between the physical and the moral.-- Testimonies, vol. 3, pp. 50, 51.

Temperance is very important to our health. We need it in every aspect of our lives. Everything needs to be in moderation.