Ad-hoc queries currently allow users to add functions to the output of the query.  For example, Count, Min, Max, Sum, Avg, and Amount.  Users can export ad-hoc queries directly, but once they associate the query with an export definition they loose the ability to include these calculations.  There are some instances where the totals are one-off and smart fields wouldn't be needed for future calculations.  There are also some instances where it's easier to have the system add these functions rather then adding them manually after the file has been exported.