Tradition, culture, etc make Celsius a useful tool. Human perception if temperatures is also not well correlated to Kelvin, where a change in 1 K is less than 0.5%, but to a person it certainly feels more substantial. By relating the scale we use daily to freezing and boiling of water, you at least capture both an okay human sensitivity, and important temperatures to us as humans.

Fahrenheit arguably goes a step further, defining a much narrower range for humans specifically, with some landmarks for water.

No system is objectively better, it's all convention and arbitrary. We could define an absolute temperature scale which puts human temperatures at 1 blorp, 0 as absolute 0. Clearly the resolution is pretty low, you'd have to define the weather with decimals. Oh well, that's fine. Annoying maybe, but valid.