So this is an incredibly general comment that might be a bit wandering, but these are my thoughts on the question:

I think I can speak a bit to this geographic area from the perspective of someone with a greater degree of specialization in North Africa (especially north-east Africa) and some professional experience with East and Southern Africa.

Obviously there are an infinite number of ways to carve up Africa, geographically.

North-to-South you might go North Africa-The Sahel-Sub-Saharan Africa. I think that division would perhaps be most troublesome in what constitutes the Sahel, and in the Horn of Africa. The Sahel is, by design, a vague term; it's not obvious where it ends. There's very little that practically distinguishes much of Chad from Libya geographically, culturally, linguistically, or tribally. Much of the vibrance of the Sahel, culturally, comes from it being a crossing between Sub-Saharan Africa and North Africa, rather than a point of focus itself.

Another area where this becomes a problem is the Horn of Africa, where for various reasons it might be plausible to group Somalia or Zanzibar, which were Islamic and at least partially Arabic-speaking if not outright Arabic, with North Africa or with the Arab cultural zone around the Red Sea, rather than with regions to their north like Ethiopia.

A north-to-south divide like this is also basically a geographic one, but misses what might be more logical cultural groupings. So "North Africa" might make sense geographically as the coast of the Mediterranean, but there are really significant cultural differences between Egypt in the east and Maghreb to the west. That's true both historically, where Islamic empires very rarely were able to control both halves of North Africa, but also down to things like colonialism and contemporary language. Morocco, Algeria, and Tunisia, among other differences, are vastly more Francophone and French-inflected than any country to their east except maybe Lebanon (which is declining rapidly.)

A similar and clear East-West divide separates regional groupings like the states of the Gulf of Guinea from the East African Community. As do additional north-south divisions like the South African Development Community. These are a mix of natural geographic groupings, cultural groupings, colonial remnants, and other rationales.

Again, this can be subdivided ad-infinitum. The old joke that "Sudan is a Continent" (pre-South Sudan's secession) because of it's incredible ethno-linguistic-tribal-religous-etc. diversity could just as easily be applied to other parts of Africa.

In broad terms I would maintain that "Sub-Saharan Africa" is quite useful for a few reasons. As I've laid out, "North Africa" is not an entirely homogenous unit, but it does have significant linguistic and cultural commonalties derived from its use of Arabic and its practice of Islam. Critically it also shares a "Mediterranean world," to borrow Ferdinand Braudel's phrase, with Europe and the Levant. Braudel's History of the Mediterranean struggled to define this world definitively, but found certain modes of life and geographic boundaries to be useful in describing a shared history, which he for simplicity's sake centers around where olives can be cultivated.

This is way beyond my expertise but I think there's also something to be said for "Sub-Saharan Africa" as "Black Africa." Race is a very fuzzy concept, but one that historically people have tried to approach as though it were concrete. Although once again there are marginal cases as one would expect from such a fuzzy issue (Are the Sudanese "black" or "Arab"?) given the long history of European colonialism and racism I think there has to be some value in delineating the primary "black" area of African society.

For more on the geographic limits at least of North Africa, see Ferdinand Braudel's The Mediterranean and the Mediterranean World.

I'm not a historian, but I have a background in Economics and Political Science, so maybe I can answer this very huge question from these perspectives. Economists have long wondered about something called the African dummy. That is to say, after taking all easily economic indicators should as current GDP per capita, commodity prices, distance from major trading partners etc countries in Sub-Saharan Africa consistently have much lower growth rates than expected.

There's a growing consensus among developmental economists, that the root cause of global poverty is poor institutions. That is to say the constitutions, written laws, informal rules and social norms that underlay economic activity. Many of the institutions of sub-Saharan Africa were set during the colonial period, and they had two big pernicious (but paradoxical effects). The first is that the African colonial state had hegemonic power. Crawford Young describes it as a Bula Matari state that could over power local elites, and required little input from the governed people. At the same time, the African Colonial state was. That is to say, there was little administrative capacity outside the capital city, and maybe some ports and mines. The sub-Saharan African Colonial state was under little pressure to either build up a powerful bureaucratic state, or build alliances with local elites. As a result you had powerful centralized states with little legitimacy or capacity. Herbst and Young emphasize that this was the result of the specific result of SSAs historical low population densities, and the context of the scramble for Africa that created these particularly anti-development colonial states, and post-colonial elites inherited an exploitative state rather than a developmental one.

None of the factors I mentioned above are unique to sub-Saharan Africa. However, it is unusual that so many countries with poor institutions are situated right next to each other. As a result, most African countries were trapped in negative spillovers rather positive externalities of trade and integration. As a result, both there was a lot of similarity in the types of economic policies pursued and their resulting political responses. A lot of countries saw the powerful faction of city based elites try to skim off the profits of globally competetive small farmers. These profits were theoretically to be used to industrialize through import substitution industrialization, but a huge chunk were misspent or appropriated by elites. As the state lost it's legitimacy (and the financial power to enforce it's dominated) outlying regions and ordinary people tried to avoid the state as much as possible. The result was slow political decay and collapse. I want to emphasize that I am painting with a giant brush here. Some countries like Ethiopia and Liberia were never colonized, and the political structures in other places like British Somaliland were very different than what I am describing here. Other states such as Kenya and Zimbabwe generally had pro-Farmer policies through the 1980s and 1990s. The level of political decay was much lower in Tanzania or Senegal than in the DRC or Chad. But I think in very broad strokes there are useful commonalities in the political development of sub-saharan Africa that make it a useful. I think there's been major divergence sinces the 2000s, but that's beyond the scope of AskHistorians.

As far as North Africa is concerned (which I admittedly know much less about) the historical evolution of the colonial and post-colonial state. However, Lisa Anderson's description of how Libya was colonized and the way the Italian colonizers created a state with access to overwhelming force, without building any sort of modern bureaucratic state is very remniscent of what happened in sub-Saharan Africa.

Troubles with the Neighbours: Africa's Problem, Africa's Opportunity, Easterly and Levine

States and Power in Africa, Jeffrey Herbst

The African Colonial State in Comparative Perspective , Crawford Young

Markets and States in Tropical Africa: The Political Basis of Agricultural Policies, Robert Bates

The State and Social Transformation in Tunisia and Libya, 1830-1980 , Lisa Anerson

Hi there,

I wrote a comment on this topic last year. I'm being a little bit polemical in it, so I would restate my fundamental point here for clarity.

I think a 2 unit categorization that separates North Africa from Sub Saharan Africa is quite troublesome. North Africa is different culturally and linguistically from West Africa or South Africa. But, those cultural and linguistic differences are not greater than Southern African differences to the Horn of Africa or West Africa. The North Africa/Sub-saharan Africa dichotomy is fundamentally built on distinctions of race, and it places very distinct cultures in one bin as "black africa".

I am of the opinion that the study of African history is better understood

1. by acknowledging that North Africa is part of the continent, and part of it's history.

2. North Africa should be one of several sub-units of study along with West Africa, Horn of Africa, East Africa, Central Africa, and Southern Africa. or an alternative categorization scheme could be divided by ecoregion (i.e. Mediterranean shore, Sahara, Sahel/savanna, West African and Congo rainforest, Horn of Africa, Lakes region, southern Savanna, Cape etc)

Edit- Also, in any categorization scheme, there should be an acknowledgement that categorizations are artificial constructs by scholars. Definitions that make sense to them, in a given context and time. Categories do not reflect hard borders, and there are lots of examples of contact and exchange between regions that wax and wane over time.