IPhone is meant to be a phone and the rest two types of iOS devices aren’t, and that’s why having a SIM card is mandatory while activating an iPhone for the first time.
However, activation of an iPhone requires a valid SIM card and you can’t activate iPhone without sim card inside the phone, like it could be done in iPads and iPod Touches. IPhones are great gadgets to begin with, and there are no doubt about the security, integrity and the features offered on these devices.
