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.