HTTPS encrypts the connection between a visitor's browser and your server, protecting data like form submissions, login details, and payment information from being intercepted. Beyond security, it's a confirmed (if modest) Google ranking signal, and Google treats HTTPS as the preferred canonical version when both HTTP and HTTPS exist.
The security part isn't just about installing an SSL certificate — done properly, it means every resource on the page loads securely (no "mixed content"), redirects are set up correctly, and nothing on your site is quietly serving a security warning to visitors or search engines.