If your site has HTTPS problems, some visitors see the words Not Secure in the address bar beside your name. Many visitors read it as a warning about the business itself.
HTTPS encrypts the connection between a visitor and your site, so what they type, a contact form, a phone number, a booking, can't be read by whoever runs the network they are on. Without it, on public wifi, someone on the same network could intercept that traffic.
Many sites have HTTPS but skip a setting called HSTS, which tells browsers to always connect securely from the first instant. Without it, the very first hop of a visit can happen unencrypted before switching over. The window is brief, but on public wifi it still exposes traffic.
Browsers warn visitors away from insecure pages, and Google factors security into how it treats a site. The label can turn away a customer who was ready to book; the fix is a settings change.
Certificates themselves are free these days; the work is in the configuration, redirects, the HSTS policy, and finding pages that still load insecure content. These are exactly the items our $40 tune-up fixes when your setup allows it.