Sim pin benefits?

I don’t see any pros to using one. iOS itself will not allow any user data of any kind to be written to the SIM, so the only use of one would be stop someone using it to make calls charged to your account. If you lose the SIM or phone, you can just contact your carrier and they’ll cancel the SIM for your account.

With modern recognitions systems (passcode + Touch or Face ID) I think PIN is not necessary.

I was using it many years ago on my iPhone 4 and I remember I experienced a lock problem after an iOS update (my carrier had to give me a new SIM card, because that iPhone wasn't able to accept my PIN code, even if it was correct).

Regards

Giulio