The poet uses a number of direct addresses to the reader. What functions do they have?

In Wordsworth's "The Solitary Reaper" (1805), he uses direct addresses to the audience to emphasize the importance of the scene. The implied audience (i.e. the reader) is to pay attention more strongly when the poet addresses them. The addresses in this poem, "Behold her, single in the field,/Yon solitary Highland Lass!," "Stop here, or gently pass!," and "O listen! for the Vale profound/Is overflowing with the sound," all take place in the first stanza of the poem. In this way, the first stanza functions as a sort of call to the reader to experience what the poet is telling us. One of Wordsworth's main focuses in his writing was the city versus country motif.
While his city poems (e.g. "Alice Fell" and "The World is Too Much with Us") offer social critiques, much the way William Blake's Chimney Sweep poems do, his nature and rural poems offer a glimpse into the sublime. The sublime is a very important concept for Romantic poets and one of the main ways it can be accessed is through the natural world. The reaper in the field, partly because she is young (see "Ode: Intimations on Immortality"), and partly because she is in nature, is closer to the sublime than those of us distracted by society. Ultimately, Wordsworth's address of the reader functions as a calling to recognize the state of the solitary reaper and share in her glimpse of the sublime.