All devices on the Web are recognized by a unique number called an IP address, for instance 123.123.123.123. When you have a site, the domain name that you type to access it is to save you time, but the server where the site files are still has an IP. Considering that there're a lot more sites and devices than there're IPs, all the shared hosting servers have a number of websites under an identical IP, whereas with a dedicated server you'll have a dedicated IP as well. Even in the first case though, you can get a dedicated IP for your sites and host them on a shared server. An advantage would be that you'll have improved search engine positions as a dedicated IP usually means a faster loading site. In addition, you need such an IP when you plan to buy an SSL certificate for your website and protect the information that your visitors submit on it.