Vitamin D drops

Idalis • 28 years old. Married to my best friend. 💍 Two beautiful kids 💗💙
I took my son to WIC today to get him all signed up and to get me changed to one of the breastfeeding packages and the nurse told me since I'm breastfeeding I need to give him vitamin D drops. Shouldn't my breastmilk have everything he needs in it? Why do I need to give him vitamin D drops? Are they necessary?