Well, dollar sign expansion is frequently used with variables but not always (as per your example). vMyDirectory is most likely a variable and you can swap in a reusable expression to be evaluated WITHIN the context of the chart you are invoking the variable with this technique. If the variable is just text like this with no preceding equals: Max(col2) then you will probably need the dollar sign expansion to have the text from the variable evaluated as an expression.
But if the text has a preding equals sign: =Max(col2) ...then qlik will likely pre-evaluate it as an expression, calculated the value OUTSIDE the context of where you are using and just insert the value. In that case, you may not need dollar sign expansion.