The reason the Julian day number is important is because Joseph Justus Scaliger went to enormous effort to create a consistent chronology of ancient and mediaeval history from records that mostly used regnal years. Sometimes they might use other calendars, or refer to the 19 year metonic cycle (determining the date of easter) or the day of the week (a 28 year cycle) or the year in the indiction cycle (a 15 year tax period in the eastern roman / byzantine empire). His chronology covered several non-european cultures as well as graeco-roman history. It is neat that astronomers are using a modernized version of his dating system, so ancient records can be more easily matched to modern observations.