Straight from their site.
NetGuard provides simple and advanced ways to block access to the internet - no root required. Applications and addresses can individually be allowed or denied access to your Wi-Fi and/or mobile connection.
Blocking access to the internet can help:
reduce your data usage save your battery increase your privacy
Edit: Uses the same slot as VPN on a phone. Thus you have one or the other. Not both. PC is different.
Then perhaps this could be a way for you to lockdown internet traffic to and from your device. Major privacy improvement.