Because we need to be able to rewrite the In clause of an AstMapExpression we need to know the AstVarExpression used for the As variable, and we need to ensure that inside the In clause that all references to the As variable use the exact same instance for the AstVarExpression that is the value of the As property so that it can be replaced using reference equality only.

This is also one of the main motivators for adding the Expression property to Symbol.