Bah I can't find the article! UGH.....
Anyway - varying terrain including different conditions like wet/muddy, dry, hard packed, sandy, etc. The workload must be comparable to the conditioning of the horse, in order for the bones and stay apparatus to successfully remodel - by that I mean you need to slightly push the comfort zone into fatigue without causing failure. In this case I'm talking fatigue of bones/ligaments not cardio, though the same holds true with cardio.
Boots - agreed good to use when support is necessary when recovering from an injury, and brushing boots (w/o support) are good to protect from interference injuries. Boots are also great to support the muscles/joints/ligaments when working to increase the payload doing conditioning work - I.e. When working outside the horse's current realm of conditioning. As the conditioning increases, the need for the boots decreases as the "normal" condition level of the horse is increased to the next level. Boots are then needed when you increase workload or work in new conditions the horse has not yet accustomed to.
Hope that make sense. I'll keep trying to find the **** article. There was one that also discusses the difference b/t US and UK racehorses (TBs in specific) where in the UK conditioning often includes gallops off the track, across varying terrain - grass, hills, dirt, mud, etc., that leads to better remodeling of the musculo-skeletal system. Whereas in the US most horses are run only in ideal conditions, frequently in track bandages, on groomed conditioned tracks, with consistant depth and texture, etc. etc. This causes the bones to not need higher density due to the lower impact surfaces and over-protection of their legs. It becomes a combination of overprotection coupled with overuse that causes the often catastrophic injuries that have become more common on the track these days. Overprotection = overuse of boots, groomed, conditioned surfaces, etc., coupled with running at young ages without allowing the bones to develop. There was actually a study done that said starting horses young was not an issue as long as they were started properly, and built up workload gradually, over varying terrain for proper remodeling. Unfortunately that's usually not the case and the work progesses at too fast a rate, in a manner that's stated above.