All devices on the Internet are identified by a special number called an IP address, for example 123.123.123.123. When you have a website, the domain name that you enter to be able to load it is for your convenience, however the server where your website files are still has an IP address. Since there are a lot more websites and devices than there are IPs, all of the shared web hosting servers have a number of sites under the same IP, whereas with a dedicated server you'll be given 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. One advantage would be that you will get better search engine positions because a dedicated IP usually means a faster loading website. What is more, you need such an IP when you intend to acquire an SSL certificate for your site and secure the data that visitors submit on it.