i work in a kitchen with loose pants so i cant keep my phone on mute, so regulating the volume based on location without having to do it manually is pretty useful for me.
same with the screen brightness; auto-dimming is useful but i also like to be able to tie certain brightnesses to certain places (i.e. in the car or when a certain app is running).
modifying the screen timeout is also useful. i tie it to my ereader so it stays on longer, messaging so if im in a conversation it stays on a little longer, if i'm in the car and it's docked it's on all the time, etc. otherwise i prefer it to turn off pretty quickly.
anytime i go to a restaurant, movie theater, library, etc. i'll add it to the Quiet Mode, then i don't even have to think about turning the phone off when i go there again. not a problem if your phone is always muted, but it's useful for me.
the low power mode isn't really to save battery on a regular basis, but i like that it's available in case my phone does get low, so it extends the usage and i dont have to rush to find a charger.