Little raised droplets of glossy paint grab illumination in the room and scatter it as specular highlights. A great effect if you're willing to invest the time in manual application.
Just out of curiosity I tried writing with my white oil-based paint marker on a scrap of Epson Enhanced Matte paper. I can hardly see the writing, depending on the angle of the light. When I can
see the writing it appears to be because the white paint is not
as white as the paper, more like a pale cream.
I get the impression that titanium dioxide would be too abrasive for use in inks for inkjet printers. I would have to agree that it's probably a far better idea to make the paper as white as possible, or white to taste, rather than introduce white ink into the system.