What is the purpose of having a Horses birthday?

It's just the same reason why you would celebrate your birthday. It's good to keep track of your horses' birthday for medical, physical etc. reasons. No one is saying yu need to have a birthday party or anything, but it is good to know the age. Racehorses' birthdays are always taken seriously and kept very good track of because they are bred to be born on January 1st and usually have to stop racing at an early age 2-3. Hope that answered your question.