Zend certified PHP/Magento developer

Understanding why is Hostname in IP address

I am trying to understand why some domain owners generate hostnames that include the IP address at which the host is found, and include those names/IPs in their DNS records. For example AWS does this with EC2 instances. This practice seems somewhat counterintuitive since the main purpose of DNS is to provide an unknown IP address for a known hostname. I can postulate a few reasons, and suspect that the explanations are organization-specific, but I have not been able to find documentation providing answers for even one case. I am looking for any suggestions for where to find such documentation, or what kinds of search terms might be helpful. Thank you in advance!