It might be useful in predicting longevity. If both parents had a lengthy and successful career, chances are they were fundamentally sound – but behind every horse is a team, and behind every spectacular horse is usually a spectacular team. Farriers, vets, handlers, and lifestyle all play a huge role in the horse’s future soundness.
When trying to look at bone density versus looking visually at a “horse with a fistful of bone”, a good example is the Arabian. They’re fine boned, light, fast moving breeds… but when you quantify their bone density, they’re on par or even better than horses significantly more substantial than them, like WBs, cobs, and some light draft breeds. Their bone density is typically fundamentally better.
The other complicated part of bone density is that it changes depending on the workload and early life limb-loading. A young horse like a thoroughbred or race bred horse, if properly exercised, will have better bone density than its non-raced equivalent, always. There is a speed component at play with increase in bone density - so it’s not just about letting a young horse move around, either. Those two year old thoroughbreds prepping for their first start, for instance, will have significantly better bone density than their WB counterparts that lounge in a stall all day.
I would say seeing photos of the parents is useful in predicting their offspring’s phenotype once mature especially for closed studbook breeds. They tend to be very homogeneous in what is produced.
Having a lengthy sport or racing career and retiring sound would be a good cornerstone to hedge on the foal’s future soundness, but not the end-all: so many management components can play into a horse’s soundness, including their farrier. So it depends. The best thing for ensuring your horse’s future soundness is ensuring your farrier is doing a good job(g) and that the horse has as little stall time as possible. So many soft tissue injuries are caused in part by bad trimming/angles and the compensation the horse endures in light of being stalled all the time.