You might have noticed that some apps, especially bank apps, present a different screen when the app goes background. Have a look at the App Switcher of your iPhone right now.
It is most certainly for privacy, but it seems to have an additional benefit: It prevents iOS from killing it.
Apparently the system will consider if your app is consuming less resources (more specifically UI resources) in the background by analyzing the top most screen of the app and it won’t knock out the app.
In my experience, I worked in a company that used to make smart doors. The doors would verify via a sensor a light pattern emitted from the app and they would open if all checks passed. Among other features, the app also had a Bluetooth connection for the convenience of opening the door touchfree and a heavy UI. Sometimes the system was killing the app and we figured out a way to keep it active in the background by adding a transparent screen every time the app was entering the background mode.
It’s a good practice not to consume unwieldy resources in the background and UI is a back-breaking demanding one. But if your app eventually needs a little bit of more stamina whilst running in the background, this might be a solution.
If you want to understand more about the App’s Life Cycle, here’s the Apple’s official documentation.