Provide (mostly) static content
This is the most obvious one: Provide a simple interface for (external) applications to display (static) content managed with Magnolia CMS.
Many custom applications have parts that consist of static content like disclaimers, imprint, documentation, faqs, etc. For those applications it's often necessary that a developer is needed to change that content or if it's possible to edit the static content it cannot be done in a convenient way (imagine editing Freemarker scripts with a text editor).
A slightly advanced use case would be to be able to easily handle variables / placeholders in the Magnolia editing environment which would be replaced by the running applications.
Having such a feature means that many new or existing applications could take advantage of adding Magnolia CMS to the mix to provide a convenient interface for non-technical people to edit static content displayed in external custom applications. Because of the REST interface it's not needed that the other applications are tightly integrated with Magnolia or even use Java.
Transferring data to a content app
Big travel companies have local field agents who are responsible for rating accommodations like hotels, etc. For that they should be able to use some of the 'modern devices' like smart phones or tablets.
They also should be able to enter some text and take photos of the objects/accommodations. A common problem these agents are facing is that there is often no local internet connection available so the text and photos
can not be edited/transferred to an M5 app/website immediately. That means you can't provide a simple solution like doing a Magnolia login on a public instance and enter the data in a M5 app.
A solution would be to have an app (not in Magnolia but on an iPhone/Android device) that is able to handle data entries offline and synchronize the data later when an internet connection is available.
For such a use case it would be great to have a simple interface to transfer the collected data to a Magnolia instance (binaries should be supported). I also can imagine that it would be a benefit that transferred data could be reviewed by an editorial team located at the travel company before showing them on a public website.
This is something that e.g. some customers want to have but I can imagine it would be a great showcase in general.