Revolutionary Cuba has pursued a uniquely active foreign policy for most of its history, largely from the state's inception in 1959 until the collapse of the Soviet Union and the onset of the Special Period in 1991. However, the degree of Cuban involvement internationally largely differed between what are identified as Cuba's first and second phases of foreign policy - the second of which includes this period of African warfare that you refer to.

In the early 1960s, Castro's foreign policy priorities were understandably in Latin America, where following a tradition of populism and leftist solidarity in the region, he encouraged and cautiously supported a series of guerilla actions across Central and South America. These small-scale conflicts were characterized by a nationalist, secular, and "populist" version of Marxism, and in many ways mirrored the same one that eventually brought Castro to power. It was therefore unsurprising that Castro chose to support them.

However, this brand of Cuban foreign policy troubled the island's new Soviet sponsors. Moscow had decided to lend extensive support the new Cuban government given that, as a Marxist regime surviving 30 miles from the United States, it was a serious "thorn in the side" of the U.S. and represented too good of an opportunity to pass up. But Cuba was the exception to Soviet foreign policy; it otherwise affirmed that Latin America was an indisputable part of the United States sphere of influence, and was concerned that extensive involvement in that region would provoke the U.S. into becoming more involved in the USSR's own "home turf" of Eastern Europe.

For this reason, the Soviet Union put the brakes on Castro's Latin American nationalist adventurism. Castro, already highly reliant on Soviet support of the Cuban economy, had no choice but to relent. This shift in Cuban foreign policy was marked by Castro's eventual declaration of support for the 1968 Soviet invasion of Czechoslovakia, an invasion that went against all the nationalist and anti-imperialist principles that he stood for. In a famously lengthy speech (even for Castro), he reluctantly endorsed the invasion as necessary to maintain global communism, while simultaneously subtly critiquing the transformation of the Soviet bloc into a single political unit.

Thus began the second phase of Cuban foreign policy, which mirrored much more closely the style of engagement and support that the Soviet Union was practicing across the Eastern Hemisphere. Castro, who understood that he needed to maintain a large and professional military so as to survive in such close proximity to his ideological enemy, became increasingly interested in Africa. This was a region largely outside the American sphere of influence, where his and the Soviet Union's interests were much more compatible.

In particular, Castro was enamored with the growing movements of African nationalism across the continent, most of which closely mirrored in ideology his own Latin American nationalist leanings. In Angola, Cuba became heavily involved in support for the People's Movement for the Liberation of Angola-Labor Party (MPLA), against the right-wing National Union for the Total Independence of Angola (UNITA). The success of the Marxists in the Angolan Civil War further emboldened Castro, and led him to commit Cuban forces to conflicts in Ethiopia and Somalia, in both cases supporting actors also backed by the Soviet Union (even though, in the case of Ethiopia's war against Eritrea, he was supporting what was objectively the imperializing and colonizing force).

Given that the African wars claimed around 2,500 Cuban lives and produced substantial economic losses on the island (due to the movement of valuable manpower and materials across the Atlantic), it's difficult to understand why Castro committed his nation to these excursions so resolutely. However, Revolutionary Cuba has rarely acted as a strictly rational state, which is part of the reason why it's such a fascinating foreign policy case to study. The African excursions were textbook examples of "wars of choice" as opposed to "wars of duty," meaning that they were voluntarily initiated in solidarity with (ostensible) liberation movements against imperialism, rather than any immediate threat to Cuban self-defense.

While they resulted in very few tangible benefits to the Cuban state, the conflicts in Angola and the Horn of Africa did bring Cuba great prestige across both the Soviet bloc and the Non-Aligned Movement, the latter of which viewed Cuba as a dependable and effective opponent of global imperialism for many decades (this perception was irreparably damaged when Cuba supported the Soviet invasion of Afghanistan, a fellow non-aligned state, in 1979). That these conflicts occurred in Africa alone was largely due to the influence of the Soviet Union, which could not abide any significant military action in Cuba's own neighborhood.

Source: Farber, Samuel. Cuba Since the Revolution of 1959 (2011), chapter 3.