Help me properly setup my internet connected d…

I saw in another thread people use sub networks for their smart devices like TVs, thermostats, etc. What’s the advantage of that and how would that help privacy?

Say you use One WiFi for some devices and another WiFi with others. My router has three WiFi access points and I don’t see how separating them would help if they all go thru the same router. Unless you guys are speaking of something different.

Found here