Prodution of Drugs
Addition of human genes to animal cells could lead to sheep that make insulin in their milk. Human insulin genes would function in the sheep's udder. Adding human genes to early sheep embryos is usually not successful. Only 1 embryo in 5 survives. Only 1 cell in 100 takes up the added gene and most do not maintain it. Therefore, it makes sense to "clone" the rare animal where the added genes have survived and functioned.

Creation of Animal Oragns for Human Transplant
Alteration of pig or sheep immune systems, by adding human genes, could reduce the possibility of organ rejection.

Animal Research on Human Diseases
e.g., cystic fibrosis, by adding human genes to animal embryos.