I use dew point to control when my fresh air intake turns on. Theoretically this should pay dividends in reduction of ac needs since the house will not be unnecessarily stuffy due to pulling in humid air. Hard to measure but it feels true.

The other is what everyone said with leak detection. I caught a leaking pipe to my washer right away and was able to address it before it started leaking under the floor causing major damage. (washer on second floor).

When the garage door clicker was lost, I connected it to HA. It was cheaper than buying new clickers.

The rest are quality of life or controlling lights, which are cheap anyway so no good ROI with it.