Honestly to know his real purpose, you would have to ask him. So as readers trying to interpret, there could be several reasons for the natural/earth imagery he employs in this scene.
I think love is one of the most basic functions of life for humanity. During this scene, these two characters attempt to escape from the complication that is their families' feud. Man has done much to try and improve his life here on this earth by building dwellings to live in, organizing societies and trying to govern those societies. All of this things distract from the beauty that is the simplicity of nature. After finding each other, I think they long for the perfection that can be found within nature.
Likewise, in the natural universe, light and darkness have come to symbolize characters, situations, and attitudes. The moon is a light but night will come to represent trouble, and Juliet notes this inconsistency of the moon's orbit. The sun is a beacon of light casting warmth on that which it touches. For Romeo, Juliet performs this fuction.