Animals use their tails in a variety of ways. They provide a source of locomotion for fish, land animals use them to brush away flies, some animals use their tails for balance, etc. But when a human develops tail, thats just nature going wrong.
Human embryos normally have a prenatal tail that measures about one-sixth of the size of the embryo itself. As the embryo develops into a fetus, the tail is absorbed by the growing body. The developmental tail is thus a human vestigial structure.
Infrequently, a child is born with a “soft tail“, which contains no vertebrae, but only blood vessels, muscles, and nerves, although there have been several documented cases of tails containing cartilage or up to five vertebrae. Cases of babies with “tails” surface occasionally. Crowds converged on temples in India to see a baby born with a “tail” in 2001.