It was designed to be only the opening and explanatory section of a philosophical poem, which was never completed.
Had it been published earlier it would have saved Wordsworth from the coldness and neglect he suffered at the hands of younger men like Shelley; it might even have made their work different from what it is.
It has made Wordsworth very clear to us now.
Wordsworth is that rarest thing amongst poets, a complete innovator.

He looked at things in a new way.

He found his subjects in new places; and he put them into a new poetic form.

At the turning point of his life, in his early manhood, he made one great discovery, had one great vision.

By the light of that vision and to communicate that discovery he wrote his greatest work.