Bodily pools of nutrients are most important during times of stress, such as an emergency, an infection or during pregnancy, in which the rate of demand for particular nutrients can only be met with adequate reserves to draw upon.
In animals that produce their own vitamin C, for example, they overproduce it during infections. We cannot produce our own vitamin C, and so have to have adequate pools, and we have to supply additional vitamin C and other nutrients at the first sign of an infection to replenish those pools.
Level 1 nutrient deficiency is a measurable decrease in nutrient pool size below the personal-and-specific optimum, and I believe is quite common even in countries with abundant food supplies.
Reserves are critical – it is no accident that malaria and schistosomiasis rampage in areas where nutrition is poor. In battling infection, every nutrient is critically important. Morbidity and mortality are higher in those whose defenses are weaker.