usage of custom attributes

There is an attribute class X that can be applied at class level. This attribute has been applied to a class Y. When the class is instantiated, the attribute applied to the class also created.

Now, my question is, why cant we directly create an instance in your class Y? What is difference between creating an instance of a class explicitly and applying attribute over the class?

Or Is there anyother concept behind writing attribute classes apart from this?

Thanx in advance.