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..

There are other "cons."

See 2.2 Why you should press Enter only once to end a paragraph

I base most of my body styles on the Body Text style which I do not base on normal. See also How styles in Microsoft Word cascade.

Well, all the other cons are silly. I think there is really only one con - the one I mention above.

using a separate "Body Text" style to prevent global changes when you modify the body text format sounds like a good idea! Maybe I'll try that.