There is only a few main reason, JMO, for buying a young prospect, versus a horse already trained, that you can try out under saddle
The horse is well bred, and relatively cheap, compared to a finished horse with that breeding, AND, you have the ability to either train that horse yourself, or the financial means to invest in training, way beyond a month
You wish to complete in some sort of young horse futurity, thus need a horse that has not been messed up, and one that is young enough to still be eligible, once trained
Beyond that, the cheapest part often, of owning a horse, is the purchase price
A horse already trained, does not need several years of feeding investment, then training money, before you can ride it