Wind chill, unlike heat index, has a basis in physical reality.
All it is is the rate of heat loss at a given temp/wind speed that equates to the rate of heat loss at a given temperature with no wind.
Very simple and elegant.
Note the temperature will never drop below the “actual” temperature. All that changes is how fast you get there.
Both wind chill and relative humidity result in physical stress on the body...both are very real even if you don’t understand the science of it or science fails to explain it plain enough.
Your body will physically react to and at the heat index numbers.
Drastically elevated heart and respiration rates from excessive sweating in oppressive heat is pretty real and very physical.
Heat Index, is a measure of how your body's cooling method of perspiration has been slowed due to higher humidity and resulting slower evaporation on your skin.
It results in an equivalent temperature where over time you could expect to experience overheating of your body. It is a real physical reality.