For the arch-Romantic Wordsworth, nature wasn't just a repository of pretty objects such as trees, mountains, birds, and flowers; it was a living force in its own right, of which humans were an integral part.
Wordsworth believed that instead of standing over against nature, man should recognize his kinship with it. He should realize that he is a part of the natural world, along with its flora and fauna.
This includes the humble butterfly, whom the speaker addresses in "To a Butterfly." Though very different creatures, the speaker and the butterfly are part of the same natural world, the same environment. Joined together in a mystical kinship, they share the same world, with its flowers, trees, and orchards.
As the poem progresses, we see that connection between the speaker and the butterfly grow as he introduces his sister. His generous offer to the butterfly to rest his weary wings on his sister's flowers is a further illustration of the powerful bond that exists between every living thing.
Here, there is no separation between man and nature, which is exactly how it should be for Wordsworth. We have the speaker, his sister, her flowers, and the butterfly that rests upon them all joined together as one in a primal unity, where all cosmetic differences between the component parts are dissolved.