What threats does a VPN protect you against?

The main reason you might find online (especially marketed by VPN providers) is that everyone can read your private data and passwords you submit without a VPN. This is mostly not true anymore though, since nowadays most websites are served via HTTPS, which encrypts all messages between you and the server.

Another threat a VPN can protect you against is being identified by your IP. Since your IP is visible to each website and most of them will put your IP into a log file, you can be identified by it if the relevant authorities demand the site and/or internet provider to do so. Keep in mind though, that a VPN doesn't completely protect you against this threat, since you can still be identified if the VPN keeps logs of your IP itself. Also, if you give any private information to a website you can also be identified by that no matter if you are using a VPN or not.