I wanted to use SPServices in an app to do all sorts of things. Since the app is in another domain (and sometimes makes cross domain calls to other site collections), the javascript proxy provided by the SP cross domain library could be very useful. Many things can be done with the built in REST interface, but not everything. And even though it's possible, I like the SPServices syntax better :-)
Some areas where this kind of app could be interesting:
Some areas where this kind of app could be interesting:
- Show list items or document templates from another site
- Create document libraries with specific settings or create other things on the host web where farm solutions aren't possible
- For a SharePoint Online Scenario where site templates and farm solutions aren't allowed, a configuration wizard can be created as an app
- etc.