I started using HA to turn lights on and off on timer while I’m out of town, so it looks like the house is occupied.

Then, because I am a nerd, I added some environmental sensors so I could see temperature & air quality.

Eventually linked the air quality sensor to a smart thermostat, so it could turn the HVAC fan on when the air is dirty & off when clean, rather than leave it on 24/7 (like the HVAC people recommend) or on ‘circulate.’ That saved around 3.5 kWh electricity every day, or $100/year, while keeping the house dust and allergen-free.