The most obvious way in which the slave trade was critical for the developlment of the Americas was in the fact that it brought huge numbers of slaves to work in plantations in the Americas. The sugar and cotton that the slaves labored to produce were extremely important to the economic development of various countries of the Americas.
For the North American colonies, the slave trade was also important for businesses that were connected to the trade, though not to slavery directly. For example, Northern firms often provided the ships that carried the slaves. This helped the overall economy of the colonies grow, even in areas where there was not that much slavery.
These were the most important ways in which the slave trade contributed positively (in an economic sense) to the development of the Americas.