As far as who, it starts at the very top and is then disseminated to subordinate units. What those units then do with the regs depends a lot on environment and mission, but by and large the orders are followed pretty precisely.

In the context of everyday wear stateside, (and I am by no stretch of the imagination a expert in military uniform regulations or their history and purpose) I would imagine it's about professionalism and uniformity. Those two concepts govern so much of what it means to be a professional soldier for a living that it's not terribly surprising it'd carry over into appearance.

In my very personal opinion, the work uniform, cammies or fatigues, is much more sacred than any other service or dress uniform. That's because it is the very last uniform many service men and women wear and deserves to be treated with a level of respect commensurate to that. In that light I can understand not wearing it out as you would with any other type of clothing, but I have absolutely no idea if that's part of the reasoning.

Other reasons may be more practical in nature. You perform your everyday job in cammies so that means if you're not in an office environment it can get flat out filthy. Perception is reality from an outsiders point of view so I think any military unit not in a combat zone would want to ensure they present the best possible image to the surrounding community they can.

Hope that helps, but if you have any more questions fell free!