True, you don't need that argument to make the case for reparations. But the left still relies heavily on it nonetheless. It's used to justify monetary reparations (as opposed to other kinds) with higher dollar figures. But more importantly, it complements the premise of black Americans as merely an oppressed, victimized, outsider class; ironically, a premise that modern leftist identity politics happens to strongly promote.

Identity politics is built around the notion of victimhood and standing outside the dominate (i.e. "white", male) social and economic system, and tuned to bolster the sympathy and guilt factor, which notions of agency and independent contribution tend to diminish. Given that narrative, it's unsurprising that when making the case for positive contributions to the shared, national identity, they're going to heavily emphasize contributions via the institution of slavery.

But to be fair, slavery and peanut butter is basically how most Americans understand the black contribution to the American story. The dominate narrative on the left, after failing[1] with peanut butter in the 20th century, simply reverted back to slavery. Of course, the black contribution and place in the American story is infinitely more complex, substantial, and pervasive than that. We just haven't figured out to how to get out of that rut.

(And one of the primary arguments against reparations is that not only would reparations not address that, it would further it. I think even many proponents, such as Ta-Nehisi Coates, would agree with the first part of that claim. Coates admits that reparations wouldn't actually fix anything; rather, he argues that achieving reparations would be the proof that we materially reconciled ourselves with and •changed* the narrative. IOW, from that perspective the solution is not in the reparations, per se, but the process of coming to an agreement on reparations. The reparations themselves are just the stakes, so people aren't just making idle, abstract commitments--talk [alone] is cheap.)

[1] Or believing that they failed. The project of creating a shared sense of inclusive history is ongoing. I cringe at proclamations of first black this or first female that. But it's the proclamations that are cringe worthy and, I think, counter-productive, not the fact itself. (The proclamations are a double-edged sword--they draw attention to particular instances of representation of a group, but the proclamation also reinforces passivity and lower status of the group.) Taking affirmative action to improve inclusion across society is extremely important, though obviously it's in tension with notions of equal treatment and so I don't necessarily agree with all forms of formal institutionalization of affirmative action. If you look at countries where an oppressed class (minority or majority) has achieved formal, institutionalized preferences--Malaysia, South Africa, Rwanda, etc--it clearly doesn't achieve the goals and even tends to solidify hierarchy and economic disparity.

Apropos Haiti, one theory for why Haiti has never properly developed is because the society strongly internalized a victimhood mentality from the very beginning. Long after slavery ended the national identity is still centered around enslavement and oppression, rather than on a positive sense of construction and cooperation. That makes the society easy prey for demagogues and avaricious elite--internal and external.