Question

why we use radius of gyration when we find moment of inertia why not simple radius of that object? ?

Answer 1

because if we use the radius of gyration, it becomes convinient as by using radius of gyration, we can assume the object to be a point mass that is rotating about the axis of rotation at a distance equal to radius of gyration
so we don't need to take the whole thing into the picture and we take the whole thing as a point mass