Question

Why do we have 2 scales in thermometer

Answer 1

For scientific purposes, temperature is measured using the Celsius scale and "thermodynamic temperature" using the KELVIN scale, which is the Celsius scale offset so that its null point is 0K = −273.15°C, or absolute zero.
This makes 0°C equal to 273.15K.

Another point of interest to you would be that there is no "Cold" - there is only "Absence of Heat".