The fact that calibration points at sea level of celsius are very easy to produce. Just need some ice and then some boiling water. And I can't even get my hands on pure ammonium chloride. Just some mixed stuff.

Yes, calibration of the thermometer. For example with sous vide cooking, it can be useful to know how accurate your equipment is since recipes are often expressed in precise temperatures.

Thanks for the example, but I'm still not sure what that has to do with the scale since you only care about a single number in that case.

I need to look up sous vide cooking. I've never heard of it before. Seems pretty interesting.

You need at least two points to determine the slope of the thermometer, 0 and 100 as freezing and boiling point of water makes that reproducible.