Why knowing the superheat and subcooling are very useful in HVAC or AC system?

The purpose is to know the enthalphy for each process i.e in condenser, compressor and evaporator. By knowing this, you can choose a suitable design for the three components.

The right anwer because we prefer AC system, because easily convert into DC.

Super heat and sub cooling are related to refrigeration status, both have impact on performance of refrigeration cycle, and on the Qadd in the evaporator and   Qrej from the condenser, the more sub cooled/super heated the water entering and leaving evaporator the more Qadd in the evaporator which maximize the performance and efficiency but on the other hand it also maximize the Qrej required to be removed in the condenser and thus maximize the condenser size and related fans or pumping power

HVAC IS Better than AC .and HVAC is not affect human body but AC some people affect in skin allergy.

Measuring superheat and sub cooling will tell us about the HVAC system performance and also we can find where is the gas in evaporator or in condenser side.

Due to following reasons, AC systems are preferred over DC systems: a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission c.

AC is more safe than DC because the current is moving 60 times per second

 superheat and subcooling it's very useful in HVAC or AC system to account the final temperature at the end of compression will depend on the working limits and the refrigerant. Taking these many factors into account, the refrigerating effect (A1 – D1 Evaporation) and the compressor energy (B1 – A1- Compression) may be read off directly in terms of enthalpy of the fluid. 

To make proper sizing of compressor and condenser

superheat and subcooling it's very useful in HVAC or AC system to account the final temperature at the end of compression will depend on the working limits and the refrigerant

ac property of raising/lowering voltage using transformers . and 3 phase cpabilities of transfering more power