Loosie, a question for you. Why do horses grow bars if we keep trimming them down? When trimming my mare's walls, the edge near the hoof wall was brought to the height of the wall but the remainder of the bar was untouched. An old farrier who had done thousands of horses told me the bars improve traction. I owned her 23 years and she was never lame.
Bars are just an extension of the rest of the walls. They grow & we trim them, just like the walls do. If there is insufficient exercise of abrasive surfaces, the bars, as with the rest, can overgrow. But not sure if that's what you meant by your question.
Perhaps you meant why do they 'pop back' so quickly if they're trimmed low? I think that if bars, quarters, etc have been overlong for some time, when they're trimmed & relieved of pressure, they 'relax down' very quickly & may suddenly become 'due for a trim' again. But
then again, I think also that when material is overtrimmed, it will often 'pop back' quickly when needed - so may be an indication that you shouldn't have gone that far. So I think both these factors need to be considered when working out how much should be trimmed, and the outcome considered carefully as to whether more or less should have been taken.
So... I tend to treat the bars like the rest of the walls - they are trimmed to the same level in relation to the sole as the rest. That depends on a number of factors, especially environment, and I do leave extra wall height(by a few mm) for traction for horses on muddy, soft ground for eg. On hard ground, walls & bars will be more likely at or hardly above the level of the sole **Always with exceptions though, esp in the heel dept - tho it seems the eg of the website I just looked at is vast overkill in that regard.
& thanks Chev!