You have completed this task if your phone explicitly asks you for the pin for your SIM card after restarting.
If your SIM card is not protected with a PIN, other people can theoretically remove it from your phone and insert it into another phone. They could then try to access your accounts that use the phone number on your SIM card. A pin protects against this very effectively. This also applies to eSims. People could theoretically try to transfer an eSim from your device to another.
This attack is similar to SIM swapping. In this case, people get your provider to send them a copy of your SIM card using your personal data.