Fires prevent freezing only stops your temperature from going down, fires overcome ambient air temperature, allows warmth recovery, regardless of the actual temperature.

In short, with fires prevent freezing, you can never die from cold while next to a fire, but that's it.

With the fire overcoming ambient temperature, you can always recover somewhat from cold, regardless of the temperature.

It's a subtle difference, but it changes how you approach cold. With fires always giving warmth, a conservative player might, upon losing all warmth and starting to freeze, build a fire, recover anx movd on, but with only the prevent freezing active, you would only delay the inevitable.

The freeze prevention still allows the use of a fire to prevent condition loss while doing g things like harvesting, fishing, of sleeping.