Why has the poet compared the nightingale's song to that of the solitary reaper's?

In line 9, the inclusion of the nightingale might be
considered to be very significant.  In starting the second stanza, the speaker,
presumably Wordsworth, has heard the song of the woman in the field.  He is enchanted
with it and cannot quite make out the meaning of the song because he doesn't understand
it, but he knows that its beauty is enchanting.  The melody is so entrancing that he is
able to project it into meaning different elements, and representing different
experiences.  The nightingale is the first image he uses to convey the beauty of the
song.  Traditionally in literature, the nightingale is one of those symbols that has
come to meant perfection.  It sings a solitary song in the dead of night, punctuating
the silence or others sounds present and within its notes lies a sense of musical
perfection that captures and expands the individual's moral and artistic imagination.
 It is this image that Wordsworth uses to compare the reaper's song to what lies in his
mind's eye, an attempt to make the subjective an external experience.  If we reflect on
it, the speaker/ Wordsworth has a daunting task.  The personal and subjective experience
of art is a challenge to convey to others.  We understand and experience art differently
than others, subjectively.  The question then becomes how can we communicate our
personal experience of art to a larger group.  How can Wordsworth/ the speaker bring to
light the "overflowing" sound of the song?  He decides that seeking to bring out images
in which others could share would be the best way to do this.  Hence, the nightingale
image.