If the user has an encrypted disk then systemd will cache the password
they type into the keyring. It makes sense to try to use this password
for automatic login purposes first, since on single user machines,
the sole user password is likely to match the disk password.

Of course if it doesn't work we'll fall back to the old way of doing
automatic login without a password (and then the user will have to
manualy enter if they need to for gnome-keyring or whatever)

https://bugzilla.gnome.org/show_bug.cgi?id=769950