It was made to fly longer routes. I have read and re-read the story in Airliners Magazine. I think I remember reading that one flew a delivery flight from Seattle to Johannesburg. That may have been a -400 but I do recall it was a very long flight.
A South African 747SP did indeed fly SEA-JNB, a distance of 8912nm, with 50 passengers aboard. This was a record for commerical aircraft till a Qantas 747-400 flew LHR-SYD, a distance of 9167nm. Both were delivery flights.