You'll find descriptions of what the mixing mediums do at
http://www.specktra.net/multi.php
I use both the water based and Face & Body mediums. The water based medium is great for turning any eyeshadow or pigment into a liquid liner with staying power. It really intensifies the colours.
The Face & Body mixing medium can be used to create custom foundations - adding irridescent powders or pigments for example and it's great when mixing two shades of Full Coverage to get an intermediate shade.