What lesson is learned from the pilot and the prince in Saint-Exupery's The Little Prince?

The most important lesson we learn from the pilot and the prince is that it is the things unseen that are most important. The pilot's drawing of a boa constrictor from the outside digesting an elephant was imperative to the pilot because it contained a secret, the enclosed elephant. The prince's fox teaches him that "what is essential is invisible to the eye" (Ch. 17). The prince looks longingly up at the sky and found the stars to be beautiful because up among the millions of stars was his own planet holding his precious flower that cannot be seen, as we see in his line, "The stars are beautiful, because of a flower that cannot be seen" (Ch. 24). When the prince and the pilot go out in search of a well, the prince calls the desert beautiful because "somewhere it hides a well" (Ch. 24). Finally, the pilot, even though the prince is physically beautiful, finds the prince to be exceptionally beautiful because of what's inside the prince, such as his heart, his wisdom, and his faithful love for a flower that can't be seen, as we see the pilot reflecting in his lines as he carries the prince through the desert in search of a well:

What I see here is nothing but a shell. What is most important is invisible ... What moves me so deeply ... is his loyalty to a flower. (Ch. 24)

Hence we see that the most important lesson the pilot and the prince teach us is that it is the things unseen, the spiritual world of love and faithfulness, that is important, rather than the corporeal world.