Was it because they wanted to extend their sphere of control or something? It seems like they were so aggressive with regard to proxy wars and spy-craft. Did they consider it a moral imperative (ie: they believed it was the correct political/economic system, therefor they felt compelled to spread Communism)?
It was not the Soviet Union that clandestinely "directed" the proliferation of communism throughout the world, even if it supported "progressive" left movements wherever they thrived. Marxism taught that the workers of the world should unite to take power away from the ruling classes of capitalism. (1) To this end socialist and communist parties were formed independently all over the world in the late nineteenth and early twentieth century, and came together three times to discuss the status of socialism in the world, what are called the First, Second, and Third Internationals.(2)
These Internationals held authority until a decade after the Russian Revolution when Stalinism became more pronounced. This divided the international proletarian parties into various schisms (schisms which preexisted the Russian Revolution also, between Bolsheviks, Mensheviks and Anarchists). Proletarian Internationalism, based on the writings of Lenin, became a buzz word in the USSR as a way of extending its influence in the world, and the USSR subsidised various peace movements as a way of increasing the membership of the proletarian parties. Many communist parties outside the USSR were directly affiliated to the Comintern, the Communist Party of the USSR, which held authority in the way the Vatican does for Catholics, even producing books and pamphlets through its own printing presses in many languages (Progress Publishers, Moscow).(3)
But other parties, such as Trotskyite, Fourth Internationalists, and then the turn to Eurocommunism by the mainstream orthodox Communist parties of Europe and Latin America in the 1970s broke ranks with the official line of the USSR, seeing the bureaucratic USSR as a form of "state capitalism" and not socialism. (4)
The development of communism as an ideology of the proletariat and workers of the whole world, has had a decisive influence on twentieth century history. Marxism flourished and provided the philosophical underpinning for revolutionaries in every corner of the world and should be seen as a global phenomenon with no real head, even if the USSR provided paternal and spiritual, sometimes economic and military support, now and then to communist movements overseas (from Africa to South America).
The motive for spreading the gospel of Marx from the USSR was both self-interested, in doing away with the real enemies that were against its very existence, and also as a way of disseminating the kerygma of socialism among the international proletariat so that all the workers of the world could finally unite and crush capitalism. Whatever we might think of such a naive ideal, it was a myth with a powerful message that found millions of dedicated adherents in the poor populations, especially in the South, during the twentieth century.
The political motives of these proletarian fighters were not well represented by the pure despotisms that flourished in the new nation states created, and run by the "dictatorship of the proletariat", quickly devolving into the "cult of personality", and a cult of the orthodox party line (to the chagrin and dismay of many European intellectuals).
(1) See Marx, Karl & Engels, Friedrich The Communist Manifesto https://www.marxists.org/archive/marx/works/1848/communist-manifesto/
(2) Braunthal, Julius (1967), History of the International 1914-1943, Vol. 2, Trans. John Clark, London: Nelson; Braunthal, Julius (1980) History of the International 1943-1968, Vol. 3, Trans. P. Ford & K. Mitchell, London: Victor Gollancz; Lindemann, A. S. (1983), A History of European Socialism, New Haven & London: Yale University Press
(3) Apart from publishing the complete works of Lenin and Stalin, this printing press also distributed the History of the Communist Party of the Soviet Union, Bolsheviks: A Short Course, designed and written by the CPSU(B) in 1938. It contained an Orthodox interpretation of its history for popular and international consumption.
(4) Abendroth, Wolfgang (1972), A Short History of the European Working Class Trans. by N Jacobs & B Trench, New Left Books: London.