I know that DVI-D is digital only and can not be transformed to VGA with passive adapter. I want to know where this adapters are used? Or is it just a marketing trick?
Example of this adapter here: http://www.amazon.com/CablesToBuy-DVI-D-Male-Female-Adapter/dp/B001CHQ52W/ref=pd_bxgy_pc_img_z/183-0695115-2473620
Best Answer
One potential intended use would be to screw two of these adapters to the each end of a VGA cable to use it as a DVI-D cable, depending on how things are connected inside. This makes little sense as two of these adapters would cost about the same as a cheap DVI cable, not to mention that it would have worse electrical characteristics. Then again, that's nothing more than a guess.
This adapter can not work as a passive DVI-I -> VGA adapter however, so the intended use cannot be that. The adapter is missing the 4 pins around the grounding pin on the side. This is where the analog red, green, blue and horizontal sync signals are transmitted. Without those pins, no DVI-A signalling is possible. The pinout is explained on Wikipedia or as a close-up here.