I had two premature babies in the NICU (twins with last minute maternal complications, everyone is fine but things were early), and they benefited so much from donor milk.

Newborns in general and preemies in particular have basically no immune system. NICU preemies are also susceptible to a very serious intestinal condition that can cause parts of their intestines to die.

Breast milk is filled with antibodies and various immune response related proteins that help bootstrap their immune system and might essentially prevent the intestinal issue entirely.

Once you’re developmentally advanced enough there’s no real long term difference between formula and breast milk, but before then the immune compounds we can’t make synthetically are basically medicine.

It’s a little odd because breast milk seems more intimate than something like blood, but it’s arguably more impactful.