Drip Cap

Drip cap is used whenever water running down a wallcan get behind a horizontal surface.

This article says that even windows with an integral nailing fin benefit from a drip cap.

https://www.marvin.com/blog/a-guide-to-window-rigid-head-flashing

This seems to be folklore, not first priciple thinking. How does a drip cap benefit if it sends water away from fully water-proof plastic?

See picture:

https://photos.google.com/search/windws/photo/AF1QipP5zbcnh7JKwpaxeR2unHZKB6H-DBuEO2eUXpPh

This is prior to sealing nailing fin with butyl tape.

Can you tell me what the drip cap is doing in this case? My conclusion is - nothing.

Properly designed windows let in water around the gaskets (but not likely at the panes themselves). Windows are designed to let this water back to the outside from the frame area:

https://www.windowguysflorida.com/blog/why-you-need-to-waterproof-your-hurricane-impact-windows/#:~:text=Due%20to%20function%20and%20design,strips%20and%20enter%20the%20frame.

Just sayin'.