I cut myself on a cheese slicer once and had to use PIN for unlocking my phone for a week or two while the cut healed. Annoying, but that's what the fallback PIN was for I guess.

Actually, the PIN isn't a fallback - the biometric is just a more convenient option once the PIN is "cached".

The biometric doesn't exist until you enter your PIN on restart, which is what unlocks the filesystem (including the biometric profile).

You'll also see that you can change your biometric with just your PIN, but you cannot use your biometric to change your PIN.