An overnight temp of 25F with no wind might not freeze a pipe because the building it is in can retain it temp above 32F all night.
But a 25F and a 10mph wind could make the building lose heat fast enough to freeze pipes before the morning warming heats it up again.
So the lower the equivalent chill temp is the more likely pipes will freeze.
The original chill index was made by measuring how long it took for a bottle of water to freeze for a given temperature and wind speed.
Wind chill - Wikipedia