How is the the growth of a poet's mind a part of "The Prelude"?
The growth of the poet's mind is not just a part of The Prelude but the driving force behind this long poem, the reason for its being. Wordsworth wanted to explain to the world how he became a poet and why he became the particular type of poet he did, one who ushered in the Romantic movement in English poetry. He planned this poem as the prelude to a longer autobiographical poem he wanted to call The Recluse.
Wordsworth dwells on the life experiences that formed his poetic sensibility. Of prime importance was the deep love of nature he formed growing up as a child in the English Lake district. He never lost this love of nature or the sense of finding the divine in the natural world. Another important influence was the French Revolution. Wordsworth happened to be in France when it broke out, and at first he was excited by the idea of forming a Republic around the brotherhood of men. He then became deeply upset and depressed after it turned into a horrendous bloodbath that betrayed all its earlier ideals. He retreated to the Lakes to heal from the trauma and gradually became aware that while he was disillusioned with overt politics, he had a vision he could impart through poetry, a vision of the common man not ridiculed as a "clown" but ennobled by being portrayed with dignity in his natural setting and described using simple, everyday language. Wordsworth also thought people could be elevated if he wrote about nature itself and the divine spark within it.
While Wordsworth rejected an obvious path at the university, which was to become a clergyman, he was influenced by religion, and especially by the dissenter poet Milton. Much of the language of his work is suffused with images of the poet as prophet. While we don't usually read long, book-length poems today, this an extraordinary work written in accessible language that is well worth pursuing, not only as an exploration of a poet's mind but because his ruminations on memory presage modernist concerns with that topic in writers such as Proust and Virginia Woolf.
I reworded the question to, in my opinion, more accurately capture its spirit. I am sorry if I made a mistake in doing so. Wordsworth writes to his sister that the purpose of his work was to detail a "poem on the growth of my own mind." It is here in which the emergence of a poetic sensibility becomes a part of "The Prelude." Wordsworth uses the poetic power of reflection and rumination to bring out his own autobiography. The poet reflects the growth of a poet's mind in the way in which the past is viewed through a Romantic sensibility. Memories of childhood are evoked that bring out innocence and a sense of beauty about what was, never to be repeated again. The reflection about university days helps to illuminate a love of nature, and a reverence for it. While the poetic sensibility explores the French Revolution, one is conscious of how the growth of a Romantic poet's mind becomes the constant and driving force behind the work. For Wordsworth, being able to explore this growth and strength of a poetic mind is as part of the significance of "The Prelude." The development poet's mind that is able to "see into the life of things" is of vital importance to the development of the poem.