Vitamin D supplement

My daughter is only breastfeed and at her 1 week check her Doctor said normally there is a recommendation you're suppose to use Vitamin D drops in their breastmilk at every feeding. I remember a few years back we used these drops with my son. I guess the drops are for their bones since mothers dont produce vitamin D. Does any use this supplement or find it neccessary? Or is breast milk just as good and they dont need it. Thoughts?