The change (in USA, anyway) to have the parking lights stay on when the head lights came on was to enable to better see a car if one of the headlights was out. Previously, if say the left headlight was burned out, a driver approaching might assume that the one lighted vehicle was a motorcycle and misjudge his position, resulting in a possible crash. If the parking lights stayed on with the headlights, an oncoming driver would realize that it was a car with a headlight out and drive appropriately, avoiding a collision. IMHO.
David: I wasn't implying that my '63 was unique. All American cars up to at least that time were wired that way.
PS: What gets my goat today is that turn signals are so close to the headlights that under certain conditions, they can't be seen clearly if the headlights are on. If they were separated from the headlights by a foot or so, they are much more visible. Once again, IMHO.