Your ISP Sells Your Personal Information
When you use the internet, you first go through your ISP, so your ISP has an exact record of which websites you've browsed, as well as potentially any sensitive data (health, financial information, etc) sent through them. And if they aren't already, your ISP can and will sell your data, due to recent rollbacks in privacy regulations.
A VPN Encrypts Data Against Your ISP
When you use a VPN, instead of going through your ISP first, your device goes through the VPN first. The VPN securely encrypts all the data between you and the app or site you're visiting, so your ISP can't see the websites you've browsed, or the personal information you send and receive through the web.