Why use potassium iodide instead of copper iodide in Sandmeyer reaction?
Sandmeyer reactions using iodides, water, and thiols do not require the copper(I) ion as a catalyst for the reaction to proceed whereas your other nucleophiles do (though not necessarily stoichiometrically). 

 



As you can see the copper(I) ion is important in reducing the diazonium to nitrogen and forming a radical.  Iodine, oxygen, and sulfur must be able to be oxidized by diazonium and therefore do not require copper.