On CDMA iPhones (Verizon, Sprint) the sim card's only purpose in life is to allow 4G/LTE to work on the phone. The sim card is tied to the phone and holds NO personal data. Leave it in and just forget it even exists.
If ever the sim card is removed, 4G/LTE will not work on a Verizon/Sprint iPhone. The user would have to go to their carrier and get a new one, for future reference in case you ever buy a used CDMA iPhone with no sim.
EDIT- I stand somewhat corrected by the post above mine. 4S iPhones it's for international use.. (and LTE if I'm not mistaken, 4S had LTE right?) Certainly on the 5 and up, it's for LTE and international.