Also, many reciples call for adding wine reducing then adding stock and reducing. Why not add it altogether and reduce once?
A: I wish I knew the answer to this - maybe it's because their pot isn't big enough. Science and common sense tell us that these steps would be nearly superfluous. And deep down, I completely agree.
Adding water to simply evaporate it back out wouldn't seem to affect anything but cooking time, and it doesn't seem as though reducing a liquid in (or not in) the presence of another liquid would change the flavor or manner in which it reduces.
BUT . . .
There are instances where I concede to what I call the "grandma factor"- this is when some ingredients, or cooking step that makes no sense at all is called for in a tried and true recipe. Bolognese certainly falls under the category of dishes inherited from another generation - along with chili, beef bourguignon, matzo ball soup, etc. etc.
I know this doesn't really qualify as an "answer" but maybe it helps you understand the question itself a little better.