When registering an SSL certificate, you can list multiple domains to be covered by it.
One use case is registering a certificate that covers both the apex domain
(example.com) and all subdomains using a wildcard (*.example.com). The
wildcard alone will not cover example.com. If you want example.com,
www.example.com, blog.example.com, etc. covered under the same certificate,
then you'll need to include both the apex and wildcard domains.