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?