The invocation of the "Arabian Sands" helps to contribute the mystery and distinctive nature of the song that the reaper sings.  The poet uses the idea when the speaker is describing of what the song reminds him and how the speaker's imagination is triggered with the notes struck in the song.  The idea of travelling in a desert, filled with sand, seeking to escape the desert's wind and heat in a small tent is contrasted with a song sung by a creature of welcoming melodious sound ("No nightingale did ever chaunt").  The image almost renders such conditions of difficulty and challenge worthwhile to the traveller through the deserts of Arabia.  Additionally, the mere invocation of the "Arabian Sands" brings to mind a world far removed of our own, where natural elements reign supreme and whose beauty and creations are rendered by a natural life force.  The sands of the desert winds are moved to make beautiful dunes, structures, and represent a spirit or power that cannot be controlled by human beings.  This life force is also representative of the song sung by the reaper.  While it is sung by a human and is the song of a human, it is far more and given her connection with the earth, it seems that her song is similar to this primordial force of life that can reach as far as "the Arabian Sands."