All devices on the Internet are identified by a unique number called an IP address, for instance 123.123.123.123. If you have a site, the domain name that you enter in order to load it is to save you time, but the server where the site files are still has an IP. Considering that there are far more sites and devices than there're IP addresses, all of the shared hosting servers have a number of sites under the very same IP, while using a dedicated server you'll have a dedicated IP too. Even in the first case though, you can get a dedicated IP for your websites and host them on a shared server. An advantage would be that you may enjoy better search engine rankings since a dedicated IP usually means a quicker loading site. Last but not least, you need such an IP when you want to get an SSL certificate for your site and protect the info that visitors submit on it.