All devices on the Web are identified by a specific number called an IP address, for instance 126.96.36.199. In case you have a site, the domain that you enter to be able to access it is for your convenience, but the server where your site files are still has an IP. As there are far more websites and devices than there're IPs, all of the shared web hosting servers have a number of websites under an identical IP, while when you use a dedicated server you'll have a dedicated IP as well. Even in the first case though, you can obtain a dedicated IP for your sites and host them on a shared server. An advantage would be that you may have superior search engine rankings since a dedicated IP generally means a quicker loading site. Last but not least, you need such an IP in case you intend to acquire an SSL certificate for your site and secure the data that visitors submit on it.