Features like this exist for putting the phone back at rest when there hasn’t been a successful unlocking for X hours – GrapheneOS, an Android OS, has a similar feature. The objective is to limit the window of time an attacker has to try to exploit anything the phone may have in operation during a not-at-rest state (when the user is still ‘logged in’ to the phone, certain background services / features may be available to exploit).

Rebooting automatically, especially if the phone not has not been successfully unlocked recently, may place the phone in a less exploitable state, as those services / features might not be available without logging in first.