Home Assistant is strange. You can't install it "normally" on a Pi because Python is too old (it wants 3.8, Raspbian has 3.7). You can install it via Docker but then you can't use Add-Ons. If you want full functionality you're really left with one choice - install their entire OS image on a dedicated Pi. For a collection of Python scripts.
They also push people strongly to us the os image. Why? Because it's going to be the best user experience for their users, easy to update, easy to use all the add-ons etc. Also, many of the low level network features, to discover IoT devices, work if you dedicate the whole pi to the platform.
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