The repetition of the moaning of the piano sets a sad, mournful tone. It only reiterates how the moaning of the piano it's just the feelings of the Blues weary musician, it is like this moan comes from the lyric voice of the piano player through the musical notes.
The sad lyrics on the excerpt contributes as well to the tone of the poem, bringing words to the melancholy of the weary singer, that after telling in the lyrics of his loneliness in the end decides to "put his troubles on the shelf"