Why are "low hours" a consideration?

I've noticed a lot of people selling items here that have "low hours", for both speakers and amplifiers. Does the sound gradually get worse over time? I haven't noticed with mine, but if it's gradual I guess I wouldn't. If I am going to spend a lot of money on a system, I would expect that if I take good care of it it should sound just as excellent years from now, and I certainly hope I shouldn't need to keep track of how many "hours" I've played it. Am I wrong in thinking this?
I would be concerned about hours for a cartridge stylus, CD/DVD laser or tubes. Other than that, I would be more concerned about deterioration due to age or environmental conditions rather than hours of use per se.

I get a big kick out of ads that refer to a non-pet environment. My dog doesn't use my stereo so I don't see what relevance that is. And the dog is a lot cleaner than some of the unsavoury characters my kids bring into the house from time to time.
LOL, surely you're joking about the dog. Dogs love hifi!
Seriously, it's in reference to pet hair and dander, which, if you've seen some people's equipment, you'd know why people use that in an ad, like smoke-free environment.