Causal vs. Constitutive

In the philosophy of science a common way to characterize the difference between kinds of explanations is in terms of the kinds of questions which they answer .... Causal theories answer questions of the form "why?" and, in some case, "how?". In providing answers to causal questions, in saying that "X causes Y," we assume .... that X and Y exist independent of each other .... that X precedes Y in time, and .... that but for X, Y would not have occurred .... Constitutive theories have a different objective, which is to account for the properties of things by reference to the structures in virtue of which they exist .... As such, constitutive questions usually take the form of "how-possible?" .... The answers to constitutive questions must support a counterfactual claim of necessity, namely that in the absence of the structures to which we are appealing the properties in question would not exist .... Constitutive theories provide explanations. These explanations are not causal, but they are explanations just the same.
Alexander Wendt, "Constitution and Causation in International Relations"

Can someone with better understanding of epistemology help me understand what the heck this is supposed to mean? I am not seeing this distinction as meaningful.