It's a commonly forgotten edge case, websites forget to normalize the hostname, the content is still served, but there's no hostname match on the browser so no cookies and broken CORS - and lots of bigger sites use a different domain to serve ads/media with a whitelist that doesn't contain the extra dot
So a machine with a search domain of 'foo.com' looking up 'www.google.com' should ideally seek the record for 'www.google.com.foo.com' first. Conversely, looking up 'www.google.com.' is more specific and returns google's ip even if 'www.google.com.foo.com' were to exist.
The reason it's not common is because all DNS client behaviors have been modified for convenience to avoid this; in basically every resolver any query containing at least one dot is automatically treated as if it were a fully qualified name. So behind the scenes, the DNS resolver is caring very much about the trailing dot and adding it to nearly every query; it's just typically hidden from users.