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First things first, let's break down what we're talking about when we say a display is "Always-On" in this context. Much like Motorola's Moto Display and Google's Ambient Display, Samsung and LG are referring to a breathing, black and white representation of information on the screen. Always-On mode will give you a glimpse at the time, recent notifications, and probably one or two other things we've not heard about yet. It means you don't have to wake the screen to see what that notification tone or vibration was, and it means you can just glance down at your phone to get that information. Samsung played with this idea last year with their edge display notifications, and LG's use of a second screen on the V10 demonstrated how they view notifications when the primary display is off. In many ways, Always-On is the natural evolution of existing display ideas.
So does an Always-On display mean more energy is consumed during the day to deliver that information to you? In most cases, no. The idea is you'll be using the breathing notification instead of constantly waking your phone, which means your phone can stay in a low-power mode to deliver you this information. If this feature only stops you from waking the phone three or four times a day, you'll still be net positive in the amount of power consumed. Also, Samsung's use of AMOLED displays means only a handful of pixels are given power to display information to you. LCD panels still have to light up the whole panel to deliver you information, but as we saw with the Moto X Pure Edition this past year the difference in power consumption is still quite low because the phone doesn't have to go from low-power to fully active to deliver you that information.
