I know you guys are into sources and you won't probably take history class as a source, so i found two wiki articles that i can link. I know it's not much better (Wikipedia is unpopular, i get it) and i know they are not in English, but i only found them in Italian. So if you know Italian, feel free to read about roman history! it.wikipedia.org/wiki/Economia_dell%27Impero_romano and it.wikipedia.org/wiki/Storia_dell%27Egitto_greco_e_romano

A bit of cart before the horse here. While Africa and Egyptian grain was a staple for the later Empire, the takeover of both was based on other, political motives entirely.

@Oldcat yes there was the whole Octavian vs. Mark Anthony business, and Cleopatra supporting him militarily. That was the reason why they marched, or rather, shipped troops there. However the reason for annexing it was its value. You might say they conquered it on a "well, since we're already here, so lets just conquer it"-logic, but that's not how roman conquests worked. There are plenty of punitive expeditions that didn't end in conquest but just did what they started out to do: punish some action or neutralize some threat. Conquest were motivated by strategical decisions...

That value was primarily just cash, not grain. It was too rich to allow anyone to take. That's why senators were not even allowed to visit Egypt by law in the Empire without permission. Still not an economic motive until later.

...be it to expand the territory to a natural border, or to obtain some advantage (tributes, access to trade routes, keeping regional powers down, access to resources, access to land for veterans). Especially during late imperial rule, they were very much aware of the difficulties to support further expansion, and weighed usefulness against costs of garrisoning and administering the territory.

@Oldcat then what motive do you suggest?

I think the motive for annexation of Egypt was political. Cleopatra showed that it was too good a base for a competitor. Rich, and hard to recapture. It would be a danger. Its importance as a granary for the cities grew later, especially after the founding of Constantinople where it became that city's supply. The original question is tough because Rome hardly ever acted for just one reason like the question wants it to.

@Oldcat Actually Egypt was the main supplier of grain for the city of Rome, shortly after it was conquered. Constantinople wasn't important until centuries and didn't surpass 500 thousand inhabitants during antiquity. Several researchers (Morris 2010 and Modelski 2003) estimate Romes population at over a million already in the first century AD, shortly after Egypt became a province. Rome needed huge amounts of grain, even before the conquest of Egypt, and nord Africa and Egypt were the main producers at the time. Before the conquest, Rome had to trade it.

trading it meant little control in case of disruptions and irregularities in the availability of goods. Supplying a city of that size and an army of that size, without having much land in the vicinity was an impressive feat.

You must consider that Italy is narrow, mountainous and prone to big weather imbalances (i'm Italian by the way).

Especially the south suffers from irregular rainfall, which can cause flooding and droughts alike.

Sicily might have been more fertile in the Antiquity, according to sources, but most agricultural centres of modern day Italy weren't habitable in Roman antiquity. Nowadays most of the crops are produced in plains (which are scarce, due to the Apennines and the Alps).

Especially the Pianura Padana (the Po valley) and the Maremma (stretching from almost Florence to around Rome along the coast). Nowadays the water has been drained and the rivers irrigate the land well, but in the antiquity those were hardly habitable marches with little agricultural output.

Now this is speculation, meaning i can't cite sources of what i'm about to say, but i think the military strategic relevance of Egypt was not so big.

Militarily, Egypt was hardly ever a threat to a major power like Rome, not before and not after. Egypts campaigns always confirmed it as a regional power at most, exerting influence no further than Syria (in very ancient Antiquity, i doubt Romans remembered that...). Further more the only natural obstacles one faces when invading Egypt are the Nile and further up towards Ethiopia some mountains.

The Nile is strategically no advantage for the defenders and the mountains are hardly relevant, because most of the population lived in the north, especially the Nile delta (Alexandria and so on...).

Having all the population concentrated along the Nile also made conquering it more easy as you could march your army straight down, without having to divide it much and you could also easily control the important areas.

Rome already had most of the middle east either directly or as a client state. They also had all of north Africa under their control, meaning Egypt would have faced war on two fronts, in case of a further war. Octavian destroyed the whole Egyptian fleet, so a seaborne invasion would not have been a problem. In the end, Egypt was no threat at all militarily, especially after it's defeat.

That's why it was always conquered. By the Hyksos, by the Greeks, by the Romans, by the Arabs, by the Byzantines again, by the Abbasids, by the Turks, by the Fatimids, followed by some internal dynasty changes, by the Ottomans again, by the French, by the British and still counting...

So it seems that Egypt is quite easily conquered.

Rome could have just made Egypt a client state, and in doing so, effectively eliminating the military threat it posed, but instead it chose to make it a province in its own right. In my opinion that can only stem from an economic consideration.

Now on what the main reason for Egypt conquest is we have to agree to disagree. But in the end the economic and political role that Egypt played was undeniable. An army needs grain long before it need silver and gold. And the great masses of people were easily swayed with grain ("panem et circenses" - bread and games). So a steady supply of staple food was of utmost importance to Rome, and logistically only achievable by owning Egypt.

note the phrase after it was conquered. We are talking the time before it was conquered, when grain for Rome primarily came from Sicily, North Africa, and Sardinia. Rome had been cut off from Egypt for decades because of the tension between Octavian and Antony.

Egypt was a huge threat to a Roman Emperor. It was a great base for a Roman competitor. It was wealthy, and hard to retake. This is why all emperors guarded access to Egypt. It was the Emperors personal property and strictly speaking not a Roman province at all! Senators were forbidden to even visit.

I never denied that Egypt was an economic asset. Just that this was not the primary reasons for the Egyptian settlement that Octavian made standing over the corpses of Antony and Cleopatra in 30 BC or so. And you can look at the wars of the Successors to Alexander to see how holding Egypt was a huge advantage to Ptolemy versus the other generals. He was very hard to get at.