A lot of systems are still paper based and I want a digital copy of everything-- my pay stubs, submitted purchase requisitions, proof of jury duty, etc. I usually use CamScanner, and think it's great, but sometimes I want to scan as quickly as possible and be done with it (when I'm submitting something on my way out of the door on a Friday, for example). Enter "Notes."
3D Touch the Notes App Icon --> Choose "Scan Document"
3D Touch is a terrible name, by the way, Apple.
My Notes app is in my Control Center, the menu you get when you swipe up from the bottom (or in the new iOS the top right hand corner). To add Notes or another app to your Control Center go to: Settings --> Control Center --> Customize Controls