I looked into this at some point and i think it was to do with using some of your RAM as extra video memory if your video card doesnt have much. Modern video cards have more than enough video memory but when MTW was first released they might not have. Some gfx chips built into motherboards might not have so much memory but I think they may use their own mechanisms for borrowing RAM anyway so this option will probably have little effect in general.

I think I recall a previous motherboard of mine having some AGP aperture size option in the BIOS settings which was essentially the same thing. I think the MTW option is a software imitation of this hardware feature.