Marx distinguished ancient from modern slavery on the basis that only the latter was generally undertaken for the production of exchange value. The exploitation of slaves on modern plantations was thus not 'restricted by more or less confined set of needs'.
"hence in antiquity over work becomes frightful only when the aim is to obtain exchange value in its independent monetary shape, i.e., in the production of gold and silver...Nevertheless, these are EXCEPTIONS IN ANTIQUITY...[t]he Negro labour in the southern states of the American Union preserved a modernately patriarchal character as long as production was chiefly directed to the satisfaction of immediate local requirements. But in proportion as the export of cotton became of vital interest ot those states, the overworking of the Negro, and soemtiems the consumption of his life in seven years of labour, became a factor in a calculated and calculating system." Capital I, p. 345 Vintage. (emphasis mine)
In my reading, modern plantation slavery is an intermediate form between pre capitalist and capitalist production. Slavery was pivotal in the creation of the world market and the primitive accumulation of capital. In many ways, modern plantation slavery was the "transition". Marx recognized this as early as Poverty of Philosophy.
This said, Marx demonstrated that a social system in which exchange value had become the general social mediation required that free and mobile wage labor be at its basis (see the first section of Immediate Process of Production which appears as an appendix to Capital I). First universal dependence on the market entails that entrepreneurs be able to undertake the production of any and all commodities; but there is little development of productive credit of which entrepreneurs can avail themselves without the availability of free, mobile and VERSATILE wage labor, that is a proletarianized labor force. The attempt by the antebellum Southern ruling class to industrialize on the basis of slave labor was unsurprisingly a failed one (see Robt Starobin Industrial Slavery in the South, which I have only skimmed). Second, the market for commodity production can only be sufficiently enlarged if the workers themselves have to satisfy their needs through the market, i.e., they have been proletarianized.
Without the destruction of slavery the capitalist mode of production could not have developed and thereby created its own free gravediggers and put in place the material foundations for communism.
At the same this raises the question, posed by Rosa Luxemburg with great force, of why capitalism reverted to unfree labor relations throughout the world in the age of imperialism.
What role do formally unfree labor relations play in the capitalist mode of production? Does the reappearance of forms of primitive accumulation imply that capitalism can never do without it, as Rosa argued (Grossmann critiqued Rosa here of course)? Are unfree labor relations under capitalism a barrier or an anomaly or at times a necessity? Never read the book by Robert Miles on the topic.