How does Dr. jekyll's letter move the plot forward

This person (who had thus, from the first moment of his entrance, struck in me what I can only describe as a disgustful curiosity) was dressed in a fashion that would have made an ordinary person laughable; his clothes, that is to say, although they were of rich and sober fabric, were enormously too large for him in every measurement-the trousers hanging on his legs and rolled up to keep them from the ground, the waist of the coat below his haunches, and the collar sprawling wide upon his shoulders ...