[Syonyk]

It was more to prevent unexpected shutdowns. Which, I'll add, were a problem with Android devices at the time, and the Nexus 5, in particular, had three battery OEMs, one of which would only last a year before being unable to run the device in high demand situations (say, "taking a picture with the flash").

As lithium batteries age, their internal resistance goes up - you can model a battery as a voltage source and a series resistor accurately enough. Over time, that resistance goes up, which means, for a given current, you end up with less voltage "at the output." Most power supplies will compensate by pulling more current to provide the needed power, which will drop the voltage more until you slam into the low voltage protection circuitry that cuts power.

The Nexus 5s are the ones I'm most familiar with, and they absolutely had this problem with one of the battery OEMs (the only way to tell which OEM you had was to pull the battery out, they were labeled on the back). The typical symptom was, "The phone shuts down when you try to take a picture," because camera modules are power hungry, the CPU was spinning hard to keep up with rendering the view from the camera (and possibly doing some pre/post frame capture to find the best frame, I don't recall when that showed up), and the flash pulls a LOT of current, very briefly. So everything would simply shut down when you hit the button to take the picture.

Apple decided to attempt to limit this problem, and they locked out the highest tiers of CPU performance (which are the most power hungry), if the device was having brownout issues. It's a reasonable enough strategy. Where they failed (IMO) was in not alerting the users that it was happening, or that it was a battery health issue. The later iterations of it, where it tracks battery health, and will tell you if your battery is going bad and needs replacement, are what they should have rolled out, and didn't. My guess is that they didn't think it was going to be a major issue for many devices, so it was just a CYA sort of thing that would prevent shutdowns. Unfortunately, that also happened right around the same time that US carriers started dropping the "New phone every 2 years on contract!" thing, and so the iPhones of that era started being used rather substantially longer than the previously-expected 2 years, and, Apple, so drama for clicks.

Had they just gone about telling users, "Hey, it looks like your battery is getting weak, would you like to schedule a replacement? Otherwise, we've limited performance slightly to prevent shutdowns." - I think it would have been fine. And they did settle on that eventually. It just took a few iterations.

[cosmic_cheese]

I experienced those shutdowns several years before that update was released when I was using an old worn out iPhone 4. Several times it died on me in the middle of an important call at 30-40% battery. I would’ve absolutely preferred it slowing itself down if it would’ve prevented that.

[CamperBob2]

If only it had occurred to Apple to ASK people what they wanted the phone to do under these conditions. What a concept!