It would be useful because it gives multiple specific and relatable reference points. How is that not useful?

The way humans relate to the temperature has a huge range and so very vague. Do you say that 0 is when you swap shorts for trousers? Or when you put a hoody on? Or is it when your neighbour puts their hoody on? Or when your friend from Texas puts their hoody on?

It's like when you come across a recipe that calls for a knob of butter. Everyone's knob is a different size, we've just agreed to say that whatever it is, it's enough.