SIM card PIN is important if you don’t want somebody to take your sim card out of your (lost) phone and use it in other phone (e.g. restore access to online/banking accounts tied to your phone number [stupidity of this practice is the whole another question]).
When encryption-enabled SFOS is booted it asks:
- LUKS pin
- SIM card pin.
SFOS should have an option to “save” SIM card PIN and input it automatically on boot after LUKS unlock, so the user is not bothered by it and also have benefits of having SIM card PIN code installed.