Freezing point being 0°C is handy because it is important to know when outdoor temperature crosses this point for multiple reasons - water is too common and important substance to ignore its properties. In Fahrenheit freezing point is a hard to remember value (for someone who used to Celsius).

I see nothing special about -17.8°C +37.78°C range except that 37.78°C is very hot. But why 37.78 and not some other value? It is arbitrary. And -17.8°C is even more arbitrary - it is not the coldest weather in many northern regions.