Burning wet wood is inefficient. A proportion of the heat is used to drive off the moisture which also reduces the temperature of the fire, this results in incomplete combustion of the wood and the gases and charcoal it produces.
Kiln dried firewood logs burn with a higher heat output and greater efficiency than wet logs, giving better fuel efficiency and fewer problems with stoves and flues.
Kiln dried hardwood firewood logs can give 3 times as much heat as the same amount of wet softwood logs
The density of softwoods is about half that of hardwoods but heat output/kg is roughly the same. Therefore it requires almost twice as many softwood logs to produce the same amount of heat as the same sized hardwood firewood logs. Softwoods also produce tars and resins that cause problems with stoves and flues.