With regard to the origins and development of the Atlantic slave trade, what ultimately led to the need for slaves?
The Europeans came to “need” slaves because they wanted work done that they were not willing or able to do themselves. They could not hire people to do the work so they decided that they needed slaves.
When the Europeans came to the Americas, they soon found that most of them could not get rich off of gold and silver the way that the Spanish in parts of Mexico and Peru were doing. The riches of the New World were to consist largely of agricultural products at this point in time. The New World was good for planting sugar and it was good for planting tobacco. Rice and indigo were also important.
All of these crops took a great deal of work and much of the work needed to be done in climates where manual labor in the middle of the day is not pleasant. In addition, there was plenty of land to be had relatively cheaply in the vicinity (at least in North America). These factors made it very hard to hire people to work the land. Most people preferred to work hard as their own masters rather than hiring out and working hard for someone else.
This meant that slaves were “needed.” There were opportunities to make money but those opportunities could not be exploited unless there was a source of labor. Slaves became that source of labor.