What does it DO! When you turn sim pin off?

The SIM PIN - when activated - prevents the SIM from functioning when first “turned on” w/o first re-entering the SIM PIN. (This includes every time your phone is turned “off” and re-started as well)

Example: A thief could steal your phone, remove your nanoSIM, insert the nanoSIM into his phone, and place/receive calls or send/receive texts AS YOU.

It also allows him to determine your phone number; making you a prime target for spear-phishing going forward. (e.g. “We found your phone; please enter your Apple credentials to confirm ownership …” )

With a SIM PIN, this could not so easily occur unless the thief ALSO knew your SIM PIN.

Use a SIM PIN for your iPhone or iPad - Apple Support

Disabling the SIM PIN does exactly what it says.

Perhaps consider the reverse - when this SIM PIN is enabled. This PIN is a security feature that prevents the SIM being used - unless the correct PIN is entered. When this feature is enabled, you must enter the SIM PIN before it will connect to the Cellular network.

Use a SIM PIN for your iPhone or iPad - Apple Support

If you consecutively enter an incorrect SIM PIN three times, the SIM will be locked and will require that you enter a PUK code that you must obtain from your Carrier. If you enter an incorrect PUK code too many times, the SIM will be permanently blocked/disabled - and must then be replaced.