As others have said, if all you want is on/off control, and one kind of light, then you probably aren't a good candidate for home automation. Or, as you say, you don't get it.
But there are many uses you haven't thought of. They're not simply for turning lights on/off on a schedule to save power.
I have some Hue lights in my living room, and, yes, I can set rainbow mode, but 90% of the time the way they're used is to light the room to different color temperatures of "white" light. In the late evening, I'll run them way down at the more yellow end of the spectrum, more like what you'd expect from candlelight, and it's a really nice way to wind down.
I have Hue lights in the bedroom and hallway, and they can provide normal lighting, but also, in the middle of the night, I can tell Siri, "set nocturnal", to get red light, dimmed to 1% brightness, in the bedroom and hallway, if I need to get up but don't wish to be jarred to full wakefulness by brighter lights.
I have an iDevices Socket hooked up to a cheap waterproof LED rope light string from Home Depot, as my porch light. After hooking it up, I spent a few minutes, one time, in the Home app's Automation tab, adding rules to turn my porch light on at sunset, off at sunrise (HomeKit knows precisely when sunset and sunrise are, every day, at your location), and starting at 9pm, it drops the brightness of the porch light by 10% each hour, until it's just a dim glow very late at night (suitable for making it up the steps and finding the keyhole, without lighting up the neighborhood). I had one of those light-sensor-controlled sockets previously, and it never worked right. Simple timers can be set accurately, but then need continual adjustment as the seasons change (an "automatic" porch light that comes on an hour after it gets dark is useless). HomeKit was a bit of overkill for this scenario, but now the porch light behaves precisely the way I want, every night without fail, automatically adjusting for differing day lengths. I don't want the light on full-blast for much of the night, or during the day, and that's not simply about saving power, I simply want it to behave the way I want.
