Why were slaves needed in the americas?

Agriculture, which was the primary means of colonial income, requires manpower. Even the more modest tobacco farms of early Virginia needed workers (indentured servitude) to function, and the rice plantations of South Carolina needed still more workers to produce any meaningful crops. In the US, this evolved into the cotton plantations of the South, while in the Caribbean the French and Spanish colonies grew sugar cane and other cash crops.

Even paying a small income to workers meant that the farms would not make any great profit. So slaves were brought from Africa to the New World to provide cheaper labor. They only needed to be provided food and limited shelter, and kept from escaping. Slaves would also have children who would add to the "free" supply of manpower.

The Spanish efforts to use native American tribes as slaves proved less successful, as they were more likely to resist, escape, or rebel. But African slaves could be just as rebellious if motivated. The colony of Haiti was lost by the French to their imported slaves in 1804 despite a large military effort.