A frozen embryo transfer is a way for horse breeders to continue to breed using a specific mare without having her go through the pregnancy. This allows for a mare to be able to produce more embryos as she does not have to go carry the foals to term. By using a surrogate mare, the foal can be birthed without any issues to the donor mare. This is extremely valuable for those mares that are entered into a lot of competitions and are priced for breeding.
Since a frozen embryo transfer allows these competition mares to continue competing after the embryo is moved, they can be bred without having to pull them from competitions or waiting until they are injured or too old to compete. The reason why they are often frozen is to allow them to be stored for a long period of time, which allows for easy transporting as well as being able to wait until the surrogate mare is ready to receive the frozen embryo transfer.