Why you add mercuric iodide in starch indicator preparation?

It gives an obvious color change. It's preparation is simple and easy plus it's not harmful or hazardous. When starch is mixed with iodine in water, an intensely colored starch/iodine complex is formed. The complex is very useful for indicating redox titrations that involve iodine because the color change is very sharp. It can also be used as a general redox indicator: when there is excess oxidizing agent, the complex is blue; when there is excess reducing agent the color disappears.