Same as others, convenience. You can entirely live without it, but after some learning curve it's not much to maintain.

I've got opening sensors on all doors and windows so my heating turns off if something is open for a few minutes.

I've got a dark hallway with some movement sensors and smart bulbs so the lights can turn on when someone walks there, with the lights being dimmed if it's late at night or not turning on if it's super late or the luminosity sensor considers it already usable (e.g. on sunny days when there's enough light bleeding in)

I've got smart bulbs in most rooms we use a lot which change the color temperature from warm to cold to warm over the course of the day depending on the sun position/time (it's a dark country, we often need lights even during the day, especially during winter)

All in all, for me it was definitely worth the price and the investment, I'd not want to go back to not having them but I imagine for someone who hasn't experienced it, it might seem superfluous or gimmicky.