The Roman Empire grew to be huge. It covered most of Europe, North Africa, and parts of Asia. Just getting messages from Rome to the outer parts of the Empire took weeks. The Empire had become too large to rule effectively. The outer provinces were pretty much doing what ever they wanted.
Emperor Diocletian was looking for a way to fix this and other problems. He decided that the only thing to do was to actually break the empire into two, a western empire one based in Rome and an eastern one based in Byzantium. There would be two emperors working together against outside enemies, but each ruling their half separately.
This turned out to be a very good plan at least for the Eastern half of the empire. When the Western Roman empire collapsed, the Eastern Empire continued for another thousand years.