The benefits of the African slave trade did not extend to millions of men, women, and children caught up in it. For them, the slave trade destroyed their families and condemned most of them to a life filled with horrors and indignities that are difficult for most in the modern Western world to imagine.
One group of beneficiaries were the monarchs and merchants within the West African kingdoms who gained wealth and prestige by participating in the trade. For them, the trade brought manufactured goods, especially guns, that added to their power, and incentivized them to participate in the trade. Over time, however, European merchants, backed by the power of their nation-states, would increasingly dominate the trade.
Also benefiting from the trade were European slave merchants who bought slaves in Africa and sold them in American markets. While different European kingdoms dominated the trade from the sixteenth to the nineteenth centuries, these merchants, many of whom enjoyed royal monopolies on the trade, profited handsomely from the sale of people. Much of the money also flowed to the financiers that provided capital for the trade, including banking houses in almost every European kingdom.
Finally, there were European people in the colonies who profited from the labor of the people ensnared in the trade. They worked to produce cash crops (especially sugar), precious metals, and other commodities that fueled the Atlantic economy. Many of these planters, notably in the West Indies, became extravagantly wealthy.
So the slave trade was immensely profitable for many of its participants. It was fundamental to the integration of the Atlantic economy in the colonial period. But all of this wealth was ultimately based on the kidnapping and sale of human beings.