I have a couple of small iPhone apps that each solve a real pain for me. They do it in a simple manner, gets out of their way so I don't have to care. I'm comfortable with the apps handling my data, and I have some control in the fact that everything is backed up to iCloud that I can restore.
When me and my brother created our mileage book app we intentionally didn't create a backend to avoid servers. All the data is stored on the device using Core Data, and is backed up to iCloud.
That means our main point of integration with other apps and services is via the phone itself. Since we don't control the data on our servers (that we don't have), we can't do integrations from there with e.g. accounting, expense or time tracking software.
But a lot of these SaaS apps also have an iPhone app and since iOS 8, app-to-app integration is natively supported without the other app exposing callback schemes.
But some SaaS apps doesn't have an iPhone app, so you can't easily invoice the time you tracked using Hours in LessAccounting. There's no way to open your timesheet when there's no iPhone app to target. They don't have an API endpoint on the device.
Hours could of course do the integration to the LessAccounting API, but there's a ton of accounting software and supporting them is a big task - so you could use Zapier as a shortcut to support more integrations.
Well, that too requires the app to integrate with Zapier, as opposed to just opening the timesheet in another accounting app on the device - so what I'm getting at here is that Zapier should make an iPhone app, like IFTTT.
It should support opening CSV, XML and JSON files. App developers still need to expose their data as said file formats, but it's a hell of a lot easier than doing a full API integration.