later edit: dear downvoters, you only get gray levels from a CT and those pretty colors assigned after what I assume is automated segmentation don't improve the diagnosis. I'd guess that any segmentation errors make the visualization more difficult.
When you have things segemented out and coloured, it is also much easier to view the object, be it an organ, blood vessel, etc. in 3 dimensions, which is something that is very useful.
Admittedly, they can get in the way sometimes, but that's why it is an optional feature.
http://media.tumblr.com/8f42d18fcc4aba25c9ded0e6d47e502b/tum...
Maybe if it was part of a spin?
Though you're right that indeed these images are using some transform between what they think bone and vascular densities and their colors.