All devices on the Internet are identified by a special number called an IP address, such as 123.123.123.123. When you have a website, the domain that you enter to be able to access it is to save you time, however the server where the site files are still has an IP. As there are far more sites and devices than there are IPs, all of the shared hosting servers have a number of websites under the same IP, whereas when you use a dedicated server you'll get a dedicated IP as well. Even in the first case though, you are able to get a dedicated IP for your sites and host them on a shared server. An advantage would be that you will get better search engine positions as a dedicated IP usually means a faster loading website. Last but not least, you need such an IP in case you intend to buy an SSL certificate for your site and secure the information that your visitors submit on it.