This unadvertised yet supremely handy feature was first spotted by MacRumors and iLounge. According to some preliminary tests, they found that although the newest iPhones still come with the same familiar 1A power supply, the iPhone 6 and iPhone 6 Plus can take advantage of the 2.1A charger included with the iPad Air to charge more quickly. It doesn’t just work with Apple-branded chargers, either: Any 2.1A charger can provide an extra bit of current to the latest iPhones, including high-wattage USB ports on newer laptops or some portable charger for iPhone.
It’s odd that Apple wouldn’t update its included power bricks or tell consumers that this feature exists. It’s most likely due to manufacturing costs. Those smaller 5W chargers cost less to manufacture than the larger 10W iPad chargers. Thus, it’s Apple’s way of saving money and giving users the smaller chargers in order to make a bit more profit off of iPhone 6 sales.
As for older iPhones, 1A chargers are still the go-to option.