The heat index measures the actual outside heat, with consideration to humidity and other factors and their effect of the temperature. This matters as it provides accurate measure of outside temperature and its impact on human body.
Heat index considers the amount of moisture or humidity in the air, which increases the temperature levels significantly. If the humidity is high, the outside temperature need not be high for the weather to be harmful for the human health. It makes it difficult for the human body to sweat, effectively cool off and minimise heat stress.