Why builder should be static inner class?

I am pretty much aware about builder pattern. Also already gone through with the builder pattern described in Item#2 in effective java of Joshua Bloch.

Here my question is - Is there any specific benefits to keep builder class inside the class which instantiates ?

We can also make it separate builder class and do the same thing.

Please be specific with your answers. AS I already know that inner class can access private members of building class and all this.