With load on it and a bright room (= display reasonably bright as well) my M3 also doesn't always last 8h. But it gets very close. Intel machines are more like 3h in that case.

Your M1 Pro is 5 years old presumably?

It’s more about not having to drag a cable + brick around everywhere and not needing to care about the location of the nearest power outlet, potentially for multiple days depending on usage patterns. You can instead leave the charging bits at home or the hotel and charge overnight.

This also means that battery cycles accumulate more slowly and the battery’s lifetime is stretched further.

Also, generally more efficient chips don’t get hot as easily which makes the laptop more comfortable to use and prevents fans from becoming audible unless you’re really pushing the system.

Probably nobody and I remember when a phone battery lasted one week: nevertheless sometimes we forgot to charge and we run out of battery. So a long lasting battery is good, too long lasting is a waste, which maybe it's your point.

However nowadays it could mean running some agents all day long (remote models) and plug in to power only at the end of the day.

It can get pretty close if avoiding Chrome and Chromium wrappers, especially in airplane mode.