post hoc, hasty generalization, ad hominem, how and why are knowledge of these fallacies useful for a writer?

short answer: So that one can use them more skillfully, and not in an inarticulate or coarse manner. Imagine a perfectly logical piece of text; is it interesting? Does it closely reflect reality? Will one be able to confront opponents from a totally different plane? If war is fought in the trenches, that's where the soldier needs to be, only with a more formidable edge (or an extra bandida of stronger armour piercing warheads.)

Because aknowedling these fallaiceis helps you break down others peoples writing/ arguments it also makes your writing stronger by avoiding them.