It's probably a hardware hack for PAL or NTSC, allowing a pixel clock to be used that is an exact harmonic of the colour subcarrier. Without that, if the video DAC's reconstruction filter doesn't have a sharp enough slope, any remnants of the pixel clock would cause quite horrible vertical banding in hue/saturation. It's probably not an issue these days, but 10+ years ago video DACs were pretty crap.

(I worked on digital video hardware many moons ago, sorry for the bizarre braindump) :-)