The 5 watt charger that comes in the box of every iPhone is identical. It was originally designed for the iPhone 4 to charge it's dinky 1420 mAh battery.
The iPhone 6+, 6S+, 7+ and 8+ all have batteries that are at least twice that capacity. The standard charger, although it works, is under powered for those big phone batteries when charging below 80%. Charging takes much longer than necessary using the dinky 5W Apple charger.
That charger is still included in the box of new big phones because IT FITS IN THE BOX. The better choice of charger to use on the newer phones is the 12 watt iPad charger.