The zeroth law of thermodynamics is essential to the existence of comparative temperature scales. Take a look at a very brief history of the three scales we use today.
At first glance, the zeroth law of thermodynamics can seem almost inane. So simple that it can be hard to see why it’s useful. But without it, and the assurance that the temperature of different bodies of matter can be consistently compared, we couldn’t have something we use every day: temperature scales.
Today’s advent offering gives a very brief history of the three temperature scales that have proven the most pervasive. Over time there have been hundreds of attempts to define temperature, but those pioneered by Fahrenheit, Celsius and Kelvin have become the ones we rely on today.
There is, of course, more to the story than we've told above. YouTuber Veritasium delves a little deeper into the history of temperature in these videos on Fahrenheit...