> Optimized Battery Charging is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged. It is available when Charge Limit is set to 100 percent. When the feature is enabled, your iPhone will delay charging past 80 percent in certain situations.
The idea is that having the battery sit at 100% charge for an extended period of time is bad for the battery. So the phone charges to 80% immediately and then to 100% a little while before it expects to be unplugged.
> Optimized Battery Charging is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged. It is available when Charge Limit is set to 100 percent. When the feature is enabled, your iPhone will delay charging past 80 percent in certain situations.
Source: https://support.apple.com/en-us/108055
The idea is that having the battery sit at 100% charge for an extended period of time is bad for the battery. So the phone charges to 80% immediately and then to 100% a little while before it expects to be unplugged.