Answer:
To communicate with enslaved people who are going to leave with her.
Step-by-step explanation:
Ann Petry's biographical work "Harriet Tubman: Conductor on the Underground" revolves around the life and work of Harriet Tubman, a former slave who helped other slaves escape and get their freedom. The famed abolitionist's work in helping other slaves gave her the nickname "Moses" among her people.
As seen in the given passage, the narrator reveals how "a goodly number of slaves would be gone from the plantation" but nobody knows how they escaped. At times, they heard some sounds similar to that of "a whippoorwill call" which was odd, for it wasn't the "season for whippoorwills". This suggests that the bird's call was used by Tubman to "communicate" with the other slaves whenever it is time for them to escape.
Thus, the reason why she used the whippoorwill sound is to communicate with the other slaves who are to leave with her and escape.