Good question! In the book The Slave Dancer by Paula Fox, Jessie does not initially realize why he has been kidnapped. However, after time on the ship, he soon ascertains his purpose—to play his pipe to make the slaves dance. Although he feels uncomfortable doing this, he fulfills his purpose due to his fear and his desire to return home.
Although Jessie also accomplishes other tasks, including killing rats, fetching items, cleaning, etc., his primary purpose is playing the pipe for the slaves. According to the crew, dancing to Jessie’s pipe will help the slaves stay healthy, which in turn will bring more profit. As Jessie’s thoughts reveal:
“It was to perform this service every other morning that I had been kidnapped and carried across the ocean.”
Thus, although Jessie does numerous activities and chores on the ship, his primary purpose is to play the pipe for the slaves.