Dew point and relative humidity, along with temperature and wind, are crucial measures to predicting how you will feel.

https://en.wikipedia.org/wiki/Dew_point

https://en.wikipedia.org/wiki/Humidity#Relative_humidity

In the US, the 100th meridian is a popular demarcation for the half of the country that experiences high humidity versus the other half that experiences low humidity. It is why 100F in Phoenix, Arizona is much more tolerable than 100F in Atlanta, Georgia.

https://en.wikipedia.org/wiki/100th_meridian_west

The "feels like" metric is more closely tied to human stress and safety than raw temperature.

In cold weather (wind chill), wind strips away the thin warm layer of air next to your skin, so you lose heat faster. Hence, "feels colder".

In hot weather (heat index), humidity slows sweat evaporation, so your body can't cool itself as effectively. Hence, "feels hotter".

So it's a lot more useful for decision-making (like what to wear, weather it is safe to run/hike, how much water you need, etc.) than the plain temperature.