Does the lock screen on a encrypted device provide different security than an unencrypted device?
Yes, the lock screen is the only barrier on an encrypted but booted Android device; Android keeps the encryption key in memory and transparently decrypts all files read by applications in the background.

Of course, that is far from ideal (iOS uses a much more complicated scheme, where some classes of encryption keys are dropped the moment the screen is locked), but it is a very simple model which is still fairly effective against many kinds of attacks.

You can find more details on the implementation and the threat model on SE crypto.