They don’t.
They’re better than fireplaces because you can drag them around, but that’s it.
(Fire isn’t dragged, though!)
Burning efficiency is mostly related to fuel dosage.
It is much better to add 1-2 two-by-fours than to dump them all.
Fire loses 1 fuel per turn unconditionally
Fire that burns items gains some fuel (for wood with <10 volume it’s 1 fuel per item per turn)
Fire that burns no items loses extra 2 fuel per fire density (small loses 2, fire 4, raging 6)
So dumping all wood on the fire at once will make it burn roughly 3 times shorter than adding wood one by one.
This isn’t intended as a realism-enhancing mechanic, it’s just that the old fire mechanics resulted in fires accumulating mystical fire energy and then burning for ages with no fuel.
It wouldn’t be hard to make it so wood stoves and fireplaces limit the fuel rate consumption. This would make them more useful than brazier+gas mask combo.