I believe the `= ""` makes this parameter 1. have a default value of empty string and 2. more importantly, makes it fit with the existing syntax for a function

I believe the `= ""` makes this parameter 1. have a default value of empty string and 2. more importantly, makes it fit with the existing syntax for a function signature that indicates the function argument is optional. If I am not mistaken, that means that the language feature can be included as simply as the newly supported 'CallerArgumentExpression' annotation without needing any change to the language specification for what defines a the correct syntax of a function signature.