Why does Shakespeare have Juliet use natural/earth imagery when talking about and to Romeo in the balcony scene?
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.
In the play "Romeo and Juliet" by William Shakespeare, the author has Juliet speak in flowery language about naturally beautiful plants and other earth imagery. In other darker places in the play, he has the character describe or imagine fusty death-ridden scenes involving skeletons, bones, dust and graves and darkness. Here, he may want to highlight the beuaty of youth, of spring, hence of flowers and the fertility of the earth. it is a reference to the youth of the sweethearts and to their sweet innocence and in sharp contrast to the withering feuding of their families.
"Alike bewitched by the charm of looks" they represent the freshness of childhood as it turns like a flower to adulthood, but will wither like a rose in family hate.
In my opinion, there are two likely reasons for this. First, it is to contrast her lines with Romeo's lines that all talk about the heavens. Second, I think it is a bit of foreshadowing.
In this scene, Romeo is talking about the moon and the stars and the sun when he talks about Juliet. By contrast, Juliet talks about flowers. I think the contrast is interesting for itself, and I think it is also possibly a way of saying that Juliet is more down to earth and less flighty than Romeo.
Second, I think her flower metaphors imply that their love will not last. Flowers by their very nature are short-lived things. So will their love be.