I have a sensor in every room. They are used to control radiator thermostats. The thermostats have their own temperature sensors of course, but can't be relied on because 1) it's literally next to the radiator, and 2) are often covered by curtains, so they're inside a hot air pocket.

HA uses the external sensor readings to set an external measured value for the radiator thermostats.

I run HA solely in my wine cellar, it's exceedingly useful to have an indoor hydrometer monitoring my conditions. I run 3 in different parts of my wine cellar.

It can replace expensive thermostat sensors. For example an Ecobee Smart Sensor two pack has an MSRP of $100. You can connect the Zigbee sensors to Home Assistant and adjust your thermostat based on your bedroom if it gets too hot when you sleep. Or turn on your whole house fan.

I've got a (generic from AliExpress) indoor Zigbee temp/hum sensor outside on the balcony inside one of those "weather station" cases[0] with the sensor raised off the bottom with a bit of plastic.

Works fine for me but I guess you might run into problems in a more exposed "watery" place (London doesn't really have extreme rain or humidity.)

[0] Like this https://www.amazon.co.uk/Technoline-TXCOVER-White/dp/B08DKZD...