I'm not sure either. It's been my habit for well over a decade.

I think what I didn't like about "space after" for the Normal style is that it affects so many other styles that don't need any space after the paragraph. Yes, you can certainly tweak each style so they don't follow the Normal style, but sometimes that's not what you always want. (For example, you may want to change the fonts in the entire document from Times New Roman to Calibri.)

Of the two evils, I think I chose to abandon "space after" and go with the more rudimentary "two paragraph breaks". Well, the only con of this method is this..