IMO more accurate presence detection. Common sensors like PIR and cheaper doppler radar types can detect when there's motion, but not if a user is present but not moving in the detection area (e.g. sleeping or sitting). There's also open source projects that can track bluetooth wearables & phones to know who specifically is in a room, but these aren't able to detect people with no devices, say guests and kids.

The preferred approach at the moment is a combination of sensors to cover motion and person detection separately, which comes at a cost, both on your pocket and the time needed to get it working suitably for your needs, or maybe one of the more expensive radar sensors like the FP2 that can detect where in a space is occupied with higher accuracy than more affordable alternatives

The thermal cam is roughly in the same price range as the FP2 - however since it has I2C, something cheap like an ESP8266 can be used to turn it into a WiFi based presence detection sensor. Something like an ESP32 could be used to turn it into a presence detector and wearable tracker, negating the need for another separate sensor entirely.

Something like this would probably be quite close to plug and play for someone DIY focused, and wouldn't have the same problem as radar being able to see through walls to different areas, although this is somewhat solved by surrounding the rear of the sensor in foil with caveats.

I'm interested in how it performs outdoors in rain though, a lot of existing affordable sensors (except PIR) struggle a little in wet conditions, with doppler based ones not detecting anything

I've had a hobby project on the back burner for quite a while using some esp8266s and thermal cameras for presence detection. Got one working pretty well in a small room.