Dark/light mode is just the most minimal version of theme support possible. It is a small bounce-back from the transition that happened about 10 years ago, from themeable pick-your-everything GUIs like winXP to our-designers-know-best unconfigurables like iOS.
It's one possible use. But the recent trend is about esthetics / usability, not energy savings. Some people prefer light some dark. It probably mostly depends on their work desk environment/lighting. Ideally apps should support both and take the cue from the system settings - we've got proper APIs for that.