Why does the poet most likely use the words “damp and brown” to describe the sands?

Depending on the work you are discussing, it could refer to the fact that when an author uses words it is for a common purpose or theme in the work. 

For example, "damp and brown" are not the most flattering words which the author could use. Therefore, it more than likely is implying a darker more depressing tone. 

The answer would be analytical reasoning.