These ‘invisible apps’ are less about the way they look or how many features they cram in and more about maximizing their usefulness to you without monopolizing your attention. - Matthew Panzerino
For me the best two examples of invisible apps on iOS are the flashlight, only useable via a toggle in the Control Center, and the build in Dictionary, accessible system wide via the selection popup.
They're both very useful apps I use on a daily basis, but which have no interface of their own. They're only reachable in a certain context.
I'd love for more apps to get this feature. Checking into Swarm via a button instead of an app for example. Or via Siri.