Dew Point is the temperature at which the air becomes completely saturated with water vapor. It is used alongside air temperature to compute relative humidity via the Clausius-Clapeyron relationship and is a standard component of METAR/SYNOP observations.
Relative Humidity is “relative” to the temperature; if the temperature rises, the RH falls even if the amount of moisture stays the same. Dew Point is an absolute measure of moisture. Industry pros search for “Dew Point trends” to track the arrival of muggy air masses, as a Dew Point above 65°F is a much more reliable indicator of human discomfort and thunderstorm potential than an RH percentage.
Under clear, calm skies, the temperature will often drop until it reaches the Dew Point, at which point condensation (fog or dew) forms and releases latent heat, slowing the temperature fall. Therefore, the evening Dew Point is a primary tool for forecasting whether a “Bust” in the nighttime low is likely.
Absolutely. High Dew Points increase the “Apparent Temperature” (Heat Index), which drives air conditioning demand. Verification of Dew Point forecasts is a “Core” task for utilities who search for “latent heat load forecasting” to ensure they have enough power to meet the cooling needs of a humid city.