Invalid use of a side-effecting operator EXECUTE STRING within a function

Qn is Why would we ever want to use exec within a function. 

U are seeing this page because u are trying to execute a string using exec within a function. 

	DECLARE @SQLString nvarchar(MAX)
	DECLARE @Result int
		SELECT @SQLString = 'SELECT MAX('+@FieldName+') from '+@TableName
		EXEC(@SQLString)
	RETURN @Result

or

SELECT @SQL = 'select @res = select count(*) from ' + @table_name;


Something like this is why i started searching for a solution for @table_name 


Why its not allowed. 
-> Reason 1 - Performance 
-> Reason 2 - Trust 

Ok Now . Research shows that at the current time UDF's can still EXEC a certain class of Stored Procedure called an Extended Stored Procedure.