The complete purpose of PreBoard is currently unknown.
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.