Transgenic animals are ones that have had genes from other animals put in their DNA.
Animals can be given the gene to make human antibodies. These can then be injected into humans to help them face an infection- instead of waiting a long time for their body to find the correct antibody and then replicate it.
Sheep and goats have been made to produce human proteins in their milk: this makes milk more beneficial to humans.
Animals could be made with organs that are similar enough to humans that they could be transplanted: this would mean there was never a shortage; it would also mean less moral questions are asked (then using human organs.)