That's the original intent, but there's nothing stopping you from enabling it whenever to try to get the absolute max possible out of your battery. Low power mode does sacrifice quite a bit of functionality by default though.

Personally, my 15 pro can do >72 hours on a single charge WITHOUT doing stuff like this, even starting at 80% (note: mostly standby, no always on display, with ~3h screen on time total). The 80% limit is definitely something I'll be using virtually all the time.

Oh sure, I put my phone in LPM sometimes when I know it's going to be a very long day. But why would someone put it in LPM when it hits 90%? Is that referring to when it charges up to 90%, or when it's in use, unplugged, and is "down to 90%"?

The latter.

For me, I find the performance/lost functionality fine: https://support.apple.com/en-ca/HT205234

On the other hand, I’ve pretty much stopped using LPM after getting a 14 Pro and getting used to the higher refresh rate of the ProMotion display; I find the slower refresh rate really jarring (I also have a non-Pro 14 which I don’t like using for the same reason).