Apple hates SIMs, and has hated them for as long as the iPhone has existed: it is known to have explored the use of fixed, non-removable SIMs in the past.
Finally, with the iPad Air 2 and mini 3, Apple has decided to start making its move by using a reprogrammable SIM that can be taken from carrier to carrier, switching networks and pricing plans through user-friendly software alone. It's called "Apple SIM." Not all carriers are on board yet. Apple SIM is still removable, and carrier-bought iPads will use regular, locked SIMs BUT it is inevitable, within a year or two, you'll probably never see a SIM card in an Apple product again.
When Apple tweaks a SIM formula, it wins. Looking back at the original iPhone in 2007, the notion of a handset with a SIM card that could only be accessed by triggering a fidgety little tray using a paper clip was madness. Yet today, many leading smartphones are using them along with replaceable batteries, something Apple introduced to the technology world.
Next there was the micro-SIM which debuted on the original iPad in 2010. At the time, it wasn't fun: Apple broke compatibility with an enormous ecosystem of GSM devices, which made sharing an account with your iPad a huge pain. Gradually carriers have started offering micro-SIMs, but you still needed a fragile, hard-to-find adapter to use those cards anywhere else.
It happened all over again with the nano-SIM standard that today's iPads and iPhones use. Again, carriers and competing phone makers had to play catch up with even 2014’s this Samsung Galaxy S5 still using the older micro-SIM.
For many customers the removable SIM is a symbol of freedom, a way to move between devices as you please. Apple's rapid progression from mini- to micro- to nano-SIM indicates their desire to be SIM free in the near future may be the on the cards sooner rather than later!