Water has one of the largest specific heats of any substance. Why is this important for the human body?

Specific heat capacity allows a substance to retain heat. My guess is that this allows the body to be tolerant to swings in temperature, to conserve energy and calories.

But I'm just guessing.

It serves to maintain the temperature at a relatively constant level, else the enzymatic reactions would go haywire.

When hot and sweaty, the sweat evaporates and the high SH provides the necessary cooling. Evaporation is the body's natural thermostat.

when temperature increases or decreases suddenly the cells are not damaged by the variations of temperature