Can someone explain the appeal of so-called "slim SIMs"? As I understand it, this allows you to load two accounts on a single device? And carriers don't like th

Can someone explain the appeal of so-called "slim SIMs"? As I understand it, this allows you to load two accounts on a single device? And carriers don't like this aspect---or is it a security concern on their part? It amuses me that these slim-SIMs, and SIM cards in general, are one of the few pieces of technology that are utterly opaque to the user and yet are so widespread. Edit: For example, I recently upgraded to an iPhone 7, at the Apple store. This required a new SIM card, but the salesperson was very careful to return the old SIM card to me. Why? What am I supposed to do with this old SIM card?