Airbrush Medium ??
 

Is this needed to add to paints like Createx, Delta, Creamcoat etc. What does it actually do? Slow the drying process?

It's used to thin the paint without breaking down the color. I never needed it for createx paints, but do use it when I spay delta paints because they tend to be on the thick side.

It can also be used to make a color a little more transparent without weakening the bond.

Its better for paint adhesion. It breaks the surface tension of water when used with water to thin those said paints