The purpose of the app is to help get (and keep) users organized.
You can scan your bills and receipts with the app, which automatically crops and sharpens them. Stack also uses text scanning capabilities that very much play to Google's core strengths to single out important details like due dates and total amounts due. It can even categorize and organize your paperwork into similar groups called, predictably enough, stacks.
As with Google's search engine, the Stack app's organizational capabilities are impressive, but they're not perfect. So the app will undoubtedly make a mistake here and there. Even given this, however, it's a fantastic and free way you can keep your important papers organized, which will make things much easier when it comes time to file those annual taxes.
Even better, it's got a backup function that's tied to Google's cloud drive service, so if you have an account, you won't ever have to worry about losing track of your receipts. If they somehow vanish from your phone, you'll be able to pull copies of what you need from your cloud drive.
Best of all, the little app is surprisingly secure. You can augment that even further by enabling biometric sign-in on the phone, so that no one can access your documents unless they scan your fingerprint in.
It's a good app, and well worth downloading and experimenting with. Kudos to Google for offering up something genuinely useful, especially at this time of year when tax preparation efforts are giving a great many people heartburn and headaches. Download your copy from the Play Store today.