Vicar of Jesus Christ, Successor of the Prince of the Apostles, Supreme Pontiff of the Universal Church, Patriarch of the West, Primate of Italy, Metropolitan Archbishop of the Province of Rome, Sovereign of the State of Vatican City, Servant of the Servants of God.
The title Supreme Pontiff originated from a title usually held by the Roman Emperor, pontifex maximus (chief priest), whose main duty was to maintain the pax deorum (peace of the gods) by regulating expiatory sacrifices (to pagan gods) as well as administering the divine law (see wikipedia).
The first few Roman emperors who were Christian starting with Emperor Constantine still held this title, converting the title by imperial law (not church law) to make them the Head of the Church (giving Constantine the authority call the Council of Nicaea in 325 AD to settle the Arian controversy!), until Emperor Gratian gave the title to Pope Damasus I. (Source: The Title Pontifex Maximus).
Since it was a title having to do with official state religion, what was the reason that Pope Damasus accepted (and the subsequent bishops of Rome retained) the title "Pontifex Maximus" and how were the rights & duties transferred / transformed? Related subquestions (to help form the answer only):
Why wasn't the title retired instead, since "Vicar of Jesus Christ" is the title derived from the authority given by Jesus?
What is the significance of the title today (see picture of the inscription "Benedict XVI, Pontifex Maximus")? What are the rights the current Pope enjoys and the duties that he needs to perform under this title that is not already covered by other titles?
When did the title become "Supreme Pontiff of the Universal Church" and what was the significance?
What does the Canon Law say about it?
Since the title originated when the Roman empire used to have an official religion (which became Christianity starting in AD 380) what is the relevance of the title in today's systems of secular governments, especially those holding the doctrine of separation of church and state?
If the title became useful as a state legal authority (as the article referenced above implies) can this title then be construed as potential interference with secular governments now that the age of Christendom has ceased?
Accepted answer should address the bold question at the minimum.