Answer:
The man who invented it doesn't want it. The man who bought it doesn't need it. The man who needs it doesn't know it. What is it?
It regulates our daily movements, but it feels no interest in our lives. It directs us when to come and go, but does not care if we pay attention. What is it?
You use it between your head and your toes, the more it works the thinner it grows.
I can be short and sometimes hot. When displayed, I rarely impress.
When set loose I fly away. Never so cursed as when I go astray.
Dark, feathery, and popular in Baltimore and fantasy books.