HTTPS has been a Google ranking factor since 2014. An unsecured site is penalized in search results and displays a warning to visitors.
The SSL certificate encrypts communications. Today, free certificates (Let's Encrypt) are available from most web hosts.
HSTS forces the browser to always use HTTPS, even if the user types "http://".
Strict-Transport-Security: max-age=31536000; includeSubDomains
All HTTP requests should be automatically redirected (301) to HTTPS.
http://site.com → 301 → https://site.com
Additional security headers protect against common attacks.
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block