Mark, it can be used for various purposes e.g. in YTD calculations where you want to show remaining balance after every transaction line.
For that, you can use report script. Store final value in a variable x. Add that to your report layout. Now, in the before.layout section of the layout you are using do curr.value+x(assuming curr.value holds the current value)
In your case it will be -
Say your layout is detail.1 and your value is X. You will do this -
final.value = final.value + X | X will be your first column and final.value will be printed as second column
Not much of a work.