In deformation theory, Cohen-Macaulay in codimension 2 is the first to be considered in higher order deformation. Does Cohen-Macaulay in codim. 2 have some good property to work with? Does it somehow measure how singular a scheme is?

I am reading Hartshorne's deformation theory, and I have difficulty to "see" why Cohen-Macaulay in codim. 2 is a good object to work with.

Thank you very much