Why would a baby born at 39 weeks need to be in the NICU after delivery?

What are some reasons that infants would need to be in the nicu if they are full term (39 weeks)?

UPDATE: I am only asking because a friend of mine just had her baby a few days ago (she is a teen mom at 17 years old) and her baby is in the nicu but she said that she doesnt want to tell anyone what happened and why he is in the nicu. But I am just so confused as to why the baby would need to be in the nicu if he wasnt premature. And he was full term?🤷‍♀️