In static ,their exists only one object.So same object will share in program. Whenever you modify it then will effect to all.
In instance type, value will be unique.it will not share to all. When you change one object value will not effect to other.
When a class is static ,you can call member variable and function directly with out create object. But in other case need to create an object, this way provide security.
Static Classes are used when there is no reason to have instances like in .net Framework Math Class.
Math class is netural to be static becuase there is no good reason to have a object of this class and then maintain the object.
Please refer: Static Classes and Static Class Members (C# Programming Guide)