According to the documentation of ignoreSnapshotOnNextApplicationLaunch: UIKit captures your app’s user interface and stores it in an image file. When your app is relaunched, the system displays this snapshot image in place of your app’s default launch image to preserve the notion that your app was still running.
My question is: I think the mechanism above is as default, but why most of the App I have tried is not like this include my App. So what should I do display the snapshot instead of the launch image?
No, by default app will display launch screen.
Above mentioned is to ignore snapshot on next launch if you are using state preservation process
See more details on state preservation process here: https://developer.apple.com/documentation/uikit/view_controllers/preserving_your_app_s_ui_across_launches?language=objc