According to Let's Encrypt, there are several use cases where an IP address cert makes more sense than a domain one:
- To access your website without a domain name, while using HTTPS
- Securing DNS-over-HTTPS (DoH) or other infrastructure services
- For the default page on hosting providers, in case someone posts the IP address instead of the domain
- Securing remote access to self-hosted services or devices
- Securing ephemeral connections within cloud hosting infrastructure, like connections between server backends (like HAProxy or other load balancers)
