Hi,
One of the scenarios for running totals usage along with grouping will be when you want to sum a column in detail section which may or may not include to check any other conditions on detail section columns.
For e.g: We have a grouping on account Number and we have date wise transaction details in detail section and Transaction amount for all positive amounts and negative amount is required separately. We can use 2 running totals on transacted amount for each day once for negative amount and other time for positive amount and that can be checked in the condition in "Evaluate" part to check whether the amount is negative or positive.
Thanks,
Raghavendra