I use it in the same way that @ThinhDinh & @Jeff_Hager described, but I also use it to increment/decrement counters (which I learned how to do from this post)
Users of my app are given “credits”. Every time they submit a form, I need to decrease their credit count. And conversely, if they delete a submission (which I allow them to do, within a certain time window), then I need to return that credit to them. They can also have different types of credits - free credits and paid credits - and I need to keep track of both. For example, if they submit using a free credit, then I need to “remember” that, just in case they come back later and delete the submission. It’s all a little fiddly, but thanks to the new compound actions feature is fully achievable without having to resort to any spreadsheet tomfoolery.