Actially, the iPhone does have a SIM card.
Look at the top of the phone between the headset jack and the power button. That invisiline slot with the hole is where it is stored.
Should you show an interest in removing the card, search google for “iPhone remove SIM” and you will find howtos as well as videos. It is part of the procedure in switching the iPhone to another carrier.
SIM cards were created to increase mobility between carriers. In the old days cell phones were built with a crystal inside that was locked to one carrier. When you switched to another carrier, you had to buy a new phone, no questions asked.
SIM cards enabled the customer to buy a device, and keep it even if they switched carriers. The process was as simple as calling in to deactivate your old SIM and ordering a new one from the new company and sticking it in your phone.