Inheritance and Static

Hi All,
I have two questions. Pretty basic, need real time examples.
1) What is the advantage of creating an object of child class of type base class(Am I mentioning correct??)
BaseClass b = new ChildClass();
When should we create BaseClass b = new ChildClass() ?? And what is the advantage over ChildClass b = new ChildClass();
2) When should I use Static 'Method'?? I know I can access static method without creating object. But when should I think of static when am creating a method?? I got an answer "Use a static method when the method does not belong to a specific object", But need how can I think in project requirement aspects (in Business Layer, in Data Layer)??

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