From The Apple Wiki
PreBoard on a iPhone 8 Plus running iOS 14.7.

The complete purpose of PreBoard is currently unknown.

This application runs right after boot (before the first device unlock) and thus data at rest protection is in place. Once the PIN is entered, the encrypted user storage becomes accessible, and the key is handed to the SEP for use when the device is unlocked via TouchID or FaceID. Because between iOS releases Apple changes data formats, this allows for user data to be migrated. It is also the UI that is shown when a device is restored via iTunes.

It is known that this app runs once upon restoring and/or updating. PreBoard is what shows the "slide to upgrade" text on a screen that looks similar to the Enter Passcode screen.

It also tracks if the user has activated their device or not. If the device is activated PreBoard will not show up.

If you launch PreBoard off of SpringBoard, it will not launch and the app will crash on iOS < 13. On iOS ≥ 13, it will either freeze the device if the device does not have a passcode, Touch ID, or Face ID enabled, or a white screen will be shown until the device authenticates with either Touch ID or Face ID. works with SpringBoard and backboardd.