With it I block all relevant outgoing and incoming connections to doubleclick, google analytics, my carrier (t-mobile), app-measurements, facebook, etc... The phone attempts connection to one of these just about every second, and I block them all.
Hopefully the Librem phones will continue to mature and become my future phone.
Internet access should be a permission (like camera, mic, etc.) unfortunately in Android it isn't the case and any app can communicate with any server. With NetGuard you can decide which app can access internet or not.
What I tend to do is: install an app → block it before running it the first time → look at every connection attempt made (usually first through ipv4) → wait a bit more until apps attempt connection through different protocol (ipv6 or U) → block anything I don't like, including ads, analytics, etc → tell NetGuard to now allow the "legit" connections to be made and start using the app.