The primary benefit of antenatal IVIG therapy is the protection it offers to the fetus against immune-mediated conditions. In cases of alloimmune thrombocytopenia, it helps increase fetal platelet counts and reduces the risk of bleeding complications. For HDFN, IVIG can mitigate the effects of maternal antibodies on fetal red blood cells, reducing the severity of anemia and the need for invasive procedures like intrauterine transfusions.