It also requires less immediate force on the part of the owners, which is also generally a lot cheaper (both in terms of hired guns and psychologically, in that an individual boss doesn't have to directly hold their employees at gunpoint in the way a plantation owner might).

Also, it creates enlarged markets for many consumer goods, even if those markets do not entirely fill whatever need they purport to be supplying.

Probably no coincidence that the Unionists had a more diverse economy with more mechanised production while the southerners were just demonic slave whippers.