Here's a real world reason why I love the faster charging of the 6+ and 7+... when I was driving from work to pick up my Apple Watch the week of release from that store in LA that had them, running Waze and playing music at the same time meant that the battery in my 6+ wasn't being charged fast enough from my standard car charger. The battery was actually running down as I drove despite being connected. I swapped to a 2.4 amp charger (like the ones used for iPads) and that never happened again.