http://en.wikipedia.org/wiki/Feature_connector
Basically it allows another card to transfer data rapidly in and out of VGA memory
3D accelerators or overlay cards sometimes used it
Edited: Actually, I also seem to recall I have a Matrox card with a TV tuner that links using the feature connector, that's one of the rare bi-directional configurations.
I have a Matrox video card that is tethered to a TV tuner/video capture card, but it uses two ribbon cables, so I don't know if either of them follow the VGA feature connector standard -- or maybe one does, and the other cable is used as the input from the TV/capture card to the video card.
A similar arrangement was used in the late '90s to connect the video card to an external MPEG2 decoder card, to allow DVD playback before computers had CPUs powerful enough to decode the DVD video in software.
Didn't the Voodoo 2 do something similar, except through the external VGA connection? It would let some other card handle the 2d work, and then take over on any 3d requirements. You would connect the 2D card's VGA out to an input on the 3D card, and then the 3D card VGA out to the monitor.