Not to mention it extends the life of the device. If a device is only just good enough for now, what will it be like in 4 years time. I currently use a 6 year old ipad air 2 and it feels very snappy and does everything I use it for well. Apple also tends to stop updates when the hardware is no longer capable of running the next OS smoothly.
In general time-to-sleep is still the defining factor in keeping battery life long. Sure, there are low-power cores, but I believe most UI code will not run at it since it's given high priority by the system.
- Less heat produced > allows for smaller heatsinks and, as a result, smaller devices or bigger components
- Performance bottlenecks become less likely
- Features like 120hz display refresh rate become possible without the user noticing degradation
- Faster OS boot and app starts
- Ability to do more work locally vs using a server (see the many ML features)
- Tech debt in system code is less of an issue - code can be shipped early and optimized later