What was Anders Celsius' greatest invention besides the Celsius thermometer?
The temperature scale that bears his name.
Kelvin is an absolute temperature, whereas both Celsius and Fahrenheit (as well as others) do not have an absolute zero value. The advantage of the Celsius scale is that one Celsius degree is the same as one degree on the Kelvin scale. As a result temperature changes are the same whether measured in Kelvin or degrees C. Besides, nobody apart from the US uses Fahrenheit anyway. Oh, yes, also Burma and Liberia.