What does a contrast medium do?

Answer 1

Some objects are impossible to image without the contrast medium. For example there's almost no difference between blood vessels or bladder and the rest of body mass. But if You fill vessel with contrast You can see the shape the contrast gets thus You can see all the problems with the vessel itself (like the change in the diameter).

Also thanks to contrast You can see the way of some liquids around the body. For example if You inject uropoline (not sure about its name in other countries) into patient's blood, after a while You'll see it in the kidneys and in bladder. If You don't - There is a problem with kidneys.

Answer 2

It makes the interior body parts more visible on an x-ray film