Think of a simple case, two pilots in currency go flying, one signs for the aircraft and takes responsibility, but the other actually does all the hand flying including the take off and landing.
After the flight, an investigator from the CAA comes sniffing round because of reports of low flying, or because the insurance was a week out of date, or the ARC was out of date, whatever. Who is the investigator going to go for?
There can certainly be a difference between the Pilot In Command from the legal sense, and the pilot flying in the physical sense, even in what is legally a single crew aircraft. It is the PIC who would have the legal responsibility for the flight, including ensuring his licence and currency was adequate for the flight he was about to undertake.