Purple is in the same category as red, blue, brown, black, yellow, etc. It's for a receptacle circuit in this photo, probably a left-over from a roll used for travellers, or home runs.
It's a commercial office. I still do residential, but rarely new residential (low-bid/cheap wiring factor), and I'm limiting the attic work as much as possible.
The metal covers sound interesting. Do they make one with a 3" nail so the rockers can see there's an outlet there?