Cooking potatoes curry
When you wash or soak cut potatoes, you are rinsing away a good deal of starch that has come to the surface during and after the cutting process (it's the white stuff that gets on your knife and fingers when cutting). 

In a potato curry, removing this starch by washing the cut pieces can result in a broth that's not as rich or thick, but I would try it both ways to see if it actually makes a notable difference - if your curry tastes watery, washing away the surface starches of the prepped potatoes is to blame.