According to LiveScience, the development of gunpowder was important in warfare first for its psychological effects, as the Chinese used explosive arrows called "flying fire" against their Mongol enemies. When the invention migrated to the West, however, its real value was in its ability to power deadly siege weapons and eventually put accurate firepower in the hands of ordinary soldiers.
Before the invention of the cannon, siege weapons were large, unwieldy and difficult to construct. Gunpowder as a propellant increased the force available for projecting missiles toward enemy emplacements, both increasing the damage these weapons could cause and increasing their accuracy as well. In addition, engineers called sappers could dig under enemy fortifications and pack their tunnels with gunpowder, taking out castle walls with explosive power. The miniaturization of cannon into firearms gave a similar destructive capability to ordinary soldiers on the ground, allowing them a much greater effective range than they had with their previous bows and crossbows. Firearms also required less training than longbows, and the high-speed projectiles could even pierce the plate armor worn by knights of the time. Ultimately, gunpowder required a complete reinvention of the art of warfare, and countries that were slow to adopt the new technology paid the price on the battlefield.