enter passcode after restart...

Print is more convenient. There's probably a better reason but that's the way I see it.

 

Could be something as simple as there way of showing you that the phone actually restarted.

 

Yes, it's for "additional security validation" per Apple.

http://support.apple.com/kb/ht5949

 

IIRC, userland is encrypted with the passcode. I wonder if inputting the passcode after restart is required to decrypt things.

 

Entering a passcode basically unwraps an encryption key that's used to secure files on your device. On pre-Touch ID devices, locking the device throws away the unwrapped key, requiring you to enter your passcode again to unlock the phone.

On Touch ID devices, the unwrapped key is handed off to the Touch ID subsystem when the device is locked. So if you restart your phone, the Touch ID subsystem has no key to provide to unlock the phone. You need to enter the passcode at least once so it has the key to unlock the phone.

The Touch ID subsystem also deletes the key after 5 unsuccessful fingerprint reads, or after 48 hours of the phone being locked. It also deletes the key if Lost Mode is activated via Find My iPhone.

This is all for security, in case your phone is stolen and they try to use a copy of your print to unlock it, or to force your finger onto the sensor to unlock your phone against your will.

Touch ID is not more secure than a complex alphanumeric passcode (although it is more secure than a 4 digit PIN), but serves as a relatively secure mechanism to prove that you're the person that knows the passcode. It's not a complete replacement, just a convenient alternative.

 

I don't mind that you have to enter your password after a restart but I hate that you have to re-enter your password for your Apple ID when you purchase an app or song.

 

Passcodes are potentially more secure. Touch ID is mainly for convenience. This is why the passcode has precedence over Touch ID after restarting the phone.

 

I wondered the same. I'm sure it's just for extra security, but it does seem unnecessary. I also get annoyed to have to manually enter my iTunes password after restarting.