To answer this question, lets look at famed Byzantine ruler Justinian. Justinian ruled the Byzantine Empire from 527-565 CE. As ruler, he faced several big problems: dealing with the Germanic tribes to the Northwest and West, establish dominance of the remains of the Roman Empire, and centralize power in Byzantium. The former two problems were what led to the development of the "Byzantine ruler".
Once the Western Roman Empire fell in 476, Germanic tribes inhabited the area. The tribes followed a different form of Christianity, treated their people with an authoritarian-like rule, and stripped Orthodox-Christians of their possessions and rights. Some Orthodox Christians approached the great Byzantine Empire and pleaded with Justinian to help his people, as "defender of the faith". It was then that Justinian went on a series of expansionist battles to reclaim the old Roman Empire. This suggests that the Byzantine ruler had to use both political shrewdness and mythology of Defender of the Faith in order to command power and rule his (or her!) empire successfully.
Not only did future Byzantine rulers need to command power at home, navigating relationships with Muslim trading cities and groups to the East (after the establishment of Islam in Mecca, 630 CE) as well as the Germanic tribes to the East and the new Kievan Rus' merchants to the North, but he (or she) had to remain the Defender of the Faith- in cases where Orthodoxy was threatened, the Byzantine ruler would be expected to intervene, as in the case of Justinian's westward expansion to incorporate the old Roman Empire (political shrewdness- more territory=more money and more power) and make sure all people in the Empire followed his codification of Roman laws to protect their rights (Defender of the Faith)