The Byzantine emperors were crowned that way to make them seem closer to god.
Step-by-step explanation:
All monarchs across the world have tried to justify their rule by the nature of divine right.
The emperor purports that they are the ones chosen divinely to rule over the dominion they have.
This is why the Byzantines wold make it an elaborate setup and and a ceremony with God to corroborate new kings so that there was a connection between the Gods and the kings.
This also made sure that the king was connected to the reign of the previous ones.