Very useful. Keeps apps I don't want on the internet from ever connecting with its firewall. Torify your whole phone, runs all internet through the Tor network and at will you can change the circuit. Also has DNScrypt so that all your web searches are encrypted and can't be monitored accurately by intrusive ovservers. Blocks ads and many more features that are damn near mandatory for privacy lovers. All while being FOSS.

You can also download from F-Droid or Droidify.