Dumbledore was the most powerful wizard in the world, and he knew it. His OWL and NEWT examiners still praised him, saying they'd seen him do things with magic that they had never dreamed of, and Dumbledore had come a long way since then. He was a genius, his mind working more quickly and more deeply than any of his colleagues', and even last year he'd been influential.
Despite all of this power, however, Dumbledore was helpless to shelter the boy he loved like a grandson: Harry Potter. He had royally messed up the whole affair with the Philosopher's Stone, and then in later years either Harry had found trouble or trouble had found Harry. It took Sirius Black's death for Dumbledore to realise that he couldn't protect Harry at all.
In the cave, after drinking that potion to get to the horcrux, Dumbledore was in agony. His throat was parched with a need for water, and he was exhausted from reliving the death of his sister. But as he watched the Inferi attack Harry, his one thought was that he must help the boy.
And as he created fire, more fire than was actually needed, he knew that he never could have dreamt of a better use for all this power than saving the Boy-Who-Lived.