When to use DD and when to use Variables?

In a template report, you can add a field to the report by selecting an alias from the Data Dictionary browser or by clicking on one of the top (Alpha, Numeric or Date) variables.

When do you use one vs the other?  Here's my thinking, if you need to operate on that variable like adding/subtracting ...etc. then use one of the variables at the top (Numeric in this case).  Otherwise select from the Data Dictionary browser.

Is that correct?  Which approach do you recommend?

Thanks