Reason to board?

I am wondering if there is a reason to board horses less than 3 years old? It seems like they can't really do anything before 3 yo and it is a waste of money to board them until then, but I buy a lot of horses already boarded at 0/1/2 yo. Am I missing something, or do people just board all without noticing/caring about the age??

I don't think it's necessary to board horses under three years old, but many do anyway.