If you sleep alone, live in a comfortable climate, and don't have any sleep problems, or back pain problems, I'm happy for you. Your experience isn't universal though and sleep is the most important thing you can do for your body so getting good sleep is paramount. Furthermore, having data on how well you slept is very useful for figuring out your own body. We wear devices to log how many steps we take, a device to log how you sleep is just an extension of that.
Are you even taking care of yourself if you don't have one?
Okay no but seriously, a smart bed that helps you get really good sleep at night so you wake up rested and ready to face the whole world may not be your cup of tea, but that's what they're selling. You could get that without all the technology, but what's the sleep company going to do with the data? Know that you sleep at night? What's the privacy danger in that?