Incendiaries and fire-starting have been used for many centuries.
Attack by fire is a fearsome thing. Not only is the heat dangerous but the smoke quickly clogs lungs. Smoke also reduces visibility and causes confusion.
Burning in camps can be of tents, equipment and the ground or woods around. This is a direct attack on soldiers and causes great fear and confusion.
Burning stores takes away vital supplies and hence causes great inconvenience, perhaps even forcing the enemy to retreat.
Burning the supply line of baggage trains removes supplies in another way, preventing vital supplies from arriving and so causing confusion and uncertainty.
Burning the arsenal deprives the enemy of their weapons. In modern warfare this causes great explosions which leads to further damage, confusion and fear.
Finally, dropping fire upon the enemy wherever they are from a height is another fearful attack on soldiers. On the battlefield, it creates smoke and confusion.