It's advertised as a scanner but for me Stack was essentially a tag/label system on top of Google Drive, something I think is still sorely needed on Google Drive. Having the same document appear in multiple "stacks" (eg "ID" + "Immigration", "Medical" + "Insurance" + "Bills") regardless of their actual folder was a good organizational tool.

Google Drive already had a fairly capable scanner.

The strange thing is that the business/education editions of Google Drive support labels mostly for compliance reasons (integrates with data retention policies), but it's not available for personal Google accounts.