Early on in his defence, Sidney identifies his thesis. Poetry is "the first light-giver to ignorance". In essence, poets set the foundation for all other learning. He states that Greece, in all her sciences, has no historical book that predates the poems of Musaeus, Homer, and Hesiod. These men were, of Greeks, "their fathers in learning". Even Plato-- who argued that poetry was merely imitation of something that was real--was a philosopher on the inside, but "the skin, as it were, and beauty depended most of poetry."
He continues to defend the significance of poetry and its writers by making Biblical reference to David's Psalms. "[W]hen [David] maketh you... see God coming in His majesty; his telling of the beasts' joyfulness, and hills leaping" it is but a heavenly poesy, and who would dare to claim that his psalms deserve to be removed from the church of God?
Poets are "the makers". They make all other learning and professions possible. Astronomers, mathmeticians, philosophers, rhetoricians, physicians, grammarians, etc. all build upon the depth of nature which, again, was grounded first by the works of the poets.