I'd like to think it adds a tone of disgust. "Horrible vermin", as in "gross creature."
Answer:
The poem is about the rain. The poet recalls sweet memories from his childhood. When the dark cloud covers the sky, and it starts raining, the poet lies down on his bed and listens to the pattern of the soft rain that falls on the roof of his home. The sound of the rain droplet brings back sweet memories of his past.
