1. They are not enabled by default
2. Their settings screen explicitly states what the modes do
3. They can extend battery life significantly
The most aggressive mode is called Ultra Stamina, and it works by basically turning the phone into dumbphone mode -- complete without any internet connectivity, calls, SMS and alarm clock only -- and the phone has no problem lasting a week.
I use these modes extensively when going backpacking for several days, as I do not have to carry any power banks, which saves precious gramms :)
edit: punctuation, grammar