A technique known as Reverse IP Lookup is a way to identify a hostname that has a DNS (A) record associated with an IP address.
A web server can be configured to serve multiple virtual hosts from a single IP address. This is a common technique in shared hosting environments. It is also common in many organizations and can be a great way to expand the attack surface during web server reconnaissance. If, for example, your primary target website appears secure, you may be able to gain access to the underlying operating system by attacking a less secure site on the same server. Potentially bypassing the target site's security controls.