After you have animals on the farm, there is a possibility that they may become sick and will need animal medicine. If an animal becomes sick, animal medicine is the only thing that will be able to cure them. If a sick animal is left uncured for long enough, they will become too sick and die. Animal Medicine works for all animals - cows, sheep and chickens.
Keeping animal medicine on hand is always a wise decision as an animal may become sick on a holiday or a day that Yodel Farm is closed.