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Independent auditors confirm top VPN doesn't log your data
(www.techradar.com)
This is a most excellent place for technology news and articles.
The most common use case is probably bittorrent. Without port forwarding, you won't be connectable. But anything where someone might need to connect to your local machine from the internet, like hosting game servers or other self-hosting.
I recently switched to Mullvad and have had no issues torrenting
You have no problem downloading because your client is initiating the connection. But people won't be able to initiate a connection to you. If you're just leeching off public trackers and don't care about your ratio, then that might not matter to you. But if you're trying to maintain a ratio on a good quality private tracker that's a no go.
You can use a site like this with your VPN ip and the port you have configured for bittorrent while your bittorrent client is up to see if you're connectable.