Building without a backend changed how I think about mobile architecture

I recently built a fairly complex mobile app without operating a traditional backend. Originally, the decision was about cost. I didn’t want to pay for servers before knowing whether people would actually use the product. That constraint forced me to rethink several things I would normally send to an API. Reading progress, bookmarks, highlights and settings are stored locally. File processing happens on the device. Backups go directly to storage controlled by the user. Even nearby file sharing temporarily turns the phone into a small local server. The unexpected result was that the app became more private, more reliable offline and less dependent on third-party infrastructure. The trade-off was development complexity. Background processing, local file conversion, device-to-device sharing and data recovery all required more work than simply sending everything to a server. I still think backends are necessary for collaboration, social features, server-side secrets, centralised accounts and real-time synchronisation. But now I ask a different question before adding one: What specific product requirement makes the server necessary? For other mobile developers here, what feature usually becomes your tipping point for introducing a backend?