The imagery of dragon fire during a clash of armies isn't just spectacle; it's a narrative tool that flips power dynamics completely. An infantry charge looks ridiculous against that kind of force, which forces characters to rely on cunning, ancient magic, or exploiting the dragon's own nature. It creates a scenario where brute strength is useless, elevating the conflict to something more cerebral or mythic.
I always think of Smaug in 'The Hobbit'. His flames don't just destroy Lake-town; they represent the unchecked, greedy power that the central quest is meant to confront. The fire is the problem incarnate, and overcoming it (through Bard's arrow at a single weak spot) requires precision and a sliver of hope, not another bigger fire. That contrast is what makes it heroic.
In some more modern takes, the flame becomes a character's burden or a corrupting element—think of a rider bonding with a dragon and struggling to control its destructive urges, where using that power risks losing themselves. The flame is both the weapon and the temptation.