Fire is required to cook Food and light structures which further process items. Food Cooked over a source of Fire lowers your Insanity a little and has a chance to give you a Gelatinous Lard.
Primary Sources of Fire
Primary sources of Fire are items that can be used to light Ignitable Structures, Items, or a Secondary Source of Fire.
Secondary Sources of Fire
Secondary sources of Fire are items or structures which after they have been lit, can be used to light other Ignitable Structures or Items.
Structures which Need Lighting
Structures which need lighting are structures which generally must be Fueled and lit in order to work.
Additionally a Frozen Mammoth must be lit in order to melt it enough to gather Mammoth Flesh and Mammoth Tusks.
Using Fire to Raise Body Temperature
Fire, when used properly, can raise your Body Temperature and contract the effects of Hypothermia. Holding or standing near specific sources of Fire will, once every 30 seconds, increase your Body Temperature.