When you connect to the Internet without a VPN, your online traffic is routed through unprotected servers of your Internet Service Provider, which means your ISP can see and log all your online activities. In some countries, ISPs are even permitted to sell online activity logs of their customers to the government and other third parties.
A VPN encrypts your online traffic and redirects it through a proprietary network of secure remote servers, making it impossible for anyone to intrude on your privacy. Not only does a VPN prevent your ISP from collecting your online activity logs, but it also protects you from such malicious actors as hackers and snoopers.