Extension Methods In Static Class???

Question

Extension methods  must be declared in non-generic, static classes. However, there is no restriction on the name of the class; you can call it whatever you want. Of course, an extension method must have at least one parameter, and only the first parameter can be marked with the this keyword. 

 

Why can't static method in non-static class Or Instance Method in non static Class be an Extension Method?? And What is the use of passing ' this'  to the first parameter??