I’m not sure why the entire phone experience needs to be slowed down by some percent for every phone of a particular model.

It’s not. The throttling is dynamic based on current battery state and current power demands. If you’re doing stuff that’s low-demand, you probably won’t be experiencing any throttling at all. If you do something demanding, only then does it slow down, and only to the extent needed. It may be as simple as dimming the screen brightness a bit while taking a video. Or maybe you’ve edited some video and need to export it. Instead of, for example, the five seconds that it would have taken, it will slow the processor down and take ten seconds. And then when you’re back to just scrolling web pages, the throttling may be gone again. The more severely degraded your battery is and the lower its charge level, the more you’ll experience throttling, but you’ll only experience throttling at the moments when, without it, your phone would have instead just shut itself off. It comes and goes as needed.