You also mention the reason finding Density Altitude (you said height, but meant Altitude)
This is used to correct the readings from an airspeed indicator to give the true airspeed of the aircraft.
The steps are:
Indicated Airspeed (subject to instrument and position error)
Calibrated Airspeed (subject to Compressibility error)
Equivalent airspeed (subject to density error)
True Airspeed (once you've factored IAS for all the above errors)