As the description says, the Vortex Monitor Master is intended to be used in conjunction with a Vortex Bridgeboard and an ISA graphics card. It switches automatically the display between the Amiga video output and the bridgeboard's VGA card output when necessary (ie. when you start bridgeboard's software the display switches to VGA, when you quit the software the display is returned to the Amiga RGB).
It is not a scandoubler and a similar function can be performed by (e.g.) the Merlin II Zorro Graphics card which has a passthrough connector. The only difference in this case would be that with the Merlin it could switch the video for you between the Amiga display and PC side graphics' card when used with Goldengate's ISA card (at least this is how I used it).
So esentially the only thing it does is eliminate the need for a manual display switch box between two outputs.