What is it used for?
The SIM card contains a unique identification number, the phone number, and other data specific to the user that it is registered to.
Without the SIM card inserted and working correctly, some phones cannot make calls, send SMS messages, or connect to mobile internet services (3G, 4G, etc)
The ability to remove the SIM card from the phone provides a significant advantage for the subscriber to switch from one phone to another. In the scenario when the existing phone is damaged, the subscriber can immediately insert the SIM card into a new phone (provided the SIM card is not damaged).