In an ideal (sim) world, a driver's health would have a progressively lower ceiling as the season progresses, but then get boosted back up to 100% (or near) between seasons to simulate the off-season break.
Of course, it's a maxim that after age 30 an athlete becomes a diminishing asset, so maybe drivers who pass 30 can no longer achieve 100% health - perhaps by age 40 the best they can get is 80% health.
But during the season, a driver's health percentage SHOULD diminish and should not, even with training, return to 100% levels. In fact, based on how hard you push a driver in a race, the percentage loss should be more significant. Again, though, a driver should be able to recover between seasons. And if you buy the Gym facility, you should be able to slow the rate of health decrease, not just unlock training.
Anyway, that's how it SHOULD work, not how it DOES (as evidenced by so many drivers in the database with 10% or lower health at age 25. :P
But, you only get max 2% bonus in training which just matches the loss of 2% in a race. There is no real benefit but to stop the degredation of your driver. Surely there should be some way of boosting your drivers health more some how.
Daniel, my driver is losing only 1% per race, so answering the question you asked in our league, I suppose the managers in our Elite tier have already trained all their staff and drivers' skills so much that they can pay more attention to their drivers' health than we can as we also have to train other skills and staff besides physical and doctor. So I guess that's how they keep their health close to 100%. I don't know why your driver is losing 2% per race, but you can train both physical and doctor at once to get +1% per race, but as I said, you better have everything else trained to 20 then :D