Answer:
Both New York City and Philadelphia have very good natural harbors, specially NYC, which has one of the best natural harbors in the whole world.
This harbors aided the economic development of the Mid-Atlantic Colonies because they facilitated the movement of goods and services, both imports and exports. It also facilitated the arrival of new migrants, and this added to the fact that the Mid-Atlantic Colonies were very open, led to a population growth that was higher than in New England or the South.
Higher populations increased the urban areas of the two cities, and it has been shown that urban economies tend to be more productive than rural economies.