The triangle allows for a longer, smoother slope on the windshield, which would probably improve aerodynamics. It's likely why the triangle is on so many hybrid cars and small, efficient hatchbacks.
Appears on other cars because it allows for a smoother transition from windshield to hood - makes the car look more streamlined.
As little as I notice it, you could accomplish all of that without the dreaded black triangle.
Edited:
I really doubt there even exists an article or "source" proving my point because nobody else cares enough to write one.
You got that right.