why we do not use farenheit scale instead of kelvin scale ?

The kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units and mostly we preferred SI units.

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Also, our understanding of gases is based on the kinetic theory of gases. This theory has to with understanding how fast gases move as a function of temperature. Since molecular motion stops at 0 K, everything is referenced to the kelvin scale, which is an absolute measurement of temperature. Celsius is just referenced to the freezing point and boiling point of water, which really has nothing to do with most gases!