Why does the Happy Prince send the ruby to the seamstress?

"The Happy Prince" is a short story written by Oscar Wilde that was published in his collection of stories entitled The Happy Prince and Other Tales in 1888. In the story, the Happy Prince is a statue who is admired by the townspeople. He was once a rich and happy prince who lived in the palace, but now, as a golden statue, he weeps over "all the ugliness and all the misery" of his city.
One day, a swallow lands on the Happy Prince and mistakes his tears for rain. Once they begin talking, the Happy Prince asks Swallow if he will deliver the ruby from his sword to a seamstress. Swallow seems burdened by the request, especially since he has plans to fly to Egypt and enjoy the warm weather with his swallow friends; however, the Happy Prince finally convinces him to help, so Swallow flies away with the ruby.
The Happy Prince chose to deliver the ruby to the seamstress because she was very poor and had a sick son that she could not provide adequate care for. Wilde describes the seamstress as having a "thin and worn" face with "coarse, red hands, all pricked by the needle."
Swallow slips into the "poor house" and places the ruby by the seamstress's thimble, then flies around her sick son to help cool his fever. When Swallow returns to the Happy Prince, he mentions that he now feels warm despite the cold weather. The Happy Prince replies, "That is because you have done a good action."