In some cases the calculated items do not allow enough flexibility to work with item values. For instance, working with text values (search/replace, string count, count matching lines, etc.) is not possible and doing more complex statistical analysis (trending) or conversions is also not possible (mapping retrieved location data to geolocation for instance).
This could all be easily solved with external script items, userparameter items or loadable module items, but we would need a way to pass the item values we want to work with to these items. Allowing the use of these macros in the key parameters will allow us to do so.