A VPN prevents your ISP from seeing exactly what websites you're visiting. Depending on your local laws and censorships, this can be either the difference in a jail sentence or a letter saying "don't download Green Day America Idiot again >:("

A VPN doesn't prevent digital fingerprinting, user accounts/profiles from being created, or dark tracking. It's entirely possible to be identified even with a VPN; you can't eat your cake and have it too.

Tl;dr you should really consider your VPN as just a shield from your ISP seeing where you go. It's not a one-click solution to anonymity, it's just one layer of many in a good opsec solution