The theatre-like structure in my opinion used to provide seating to the clergy, synthronon (set of stepped seats), in later churches (7th century and on) they would move rather to the choir. A viewer let me know That is correct. The Bishop would sit at the top in the center, and preach from there. The use of this would disappear beginning around 400 AD, with St John Chrysostom in Constantinople (he would preach from the middle of the church), and would spread through out the empire from there