Egypt was the most valuable province of Rome for two reasons. The first is obvious, in a time when any food surplus was highly valued Egypt was the bread basket of the Mediterranean world, churning out a regular, highly dependable surplus of wheat.
Secondly it operated out of step with the Northern summer season. The monsoons hit Ethiopia in the Summer causing the Nile flood, so the Egyptians were planting when the Italians and Greeks were harvesting. This allowed the Empire to stagger the deployment of transport. Ships that transported grain from Sicily and Africa in Autumn could switch to the Egyptian trade in Spring.
When Rome lost Italy, Sicily, North Africa, Sardinia and ruled from Constantinople Egypt gained in importance.
As a result the 6th of July was a black day for the Romans when, in 640 AD a small force of Arabs under the brilliant general Amr ibn al-As al-Sahmi routed the Byzantines at the battle of Heliopolis on the outskirts of Cairo.
The Romans had, after a lifetime of war by Emperor Heraclius, defeated their arch nemesis, the Sassanid Empire, in 622. As the two punch-drunk empires reeled away from each other the newly unified Arabs exploded out of the Arabian Peninsula and overran the Sassanid lands; the ancient Persian Empire.
The Romans believed themselves safe for at least a generation as the Arabs assimilated the feuding elements of the Persian empire. They met the Arabs properly for the first time at the battle of Yarmouk in Syria in a battle that lasted for six days. Rome lost Syria, but that was not a complete disaster. Rome could survive without troublesome Syria.
But Egypt was another matter. The loss of Egypt was a near deathblow to the Roman Empire. Ultimately the Byzantine Empire could only survive by re-organisation of the entire economy. The grain dole that marked out the highs of Roman Civilization had to cease when Egypt was lost. Roman dominance of Mare Nostrum, the Mediterranean Sea ended as the Arabs gained a coastline with well defended harbours.
The Arabs by contrast, were unleashed. Their cavalry thundered across the North African Deserts to Morocco and Spain. Where horses and camels galloped the ships followed. The failings of the Byzantines at Heliopolis were felt by Christians across the entire Western World.