All devices on the Web are recognized by a special number called an IP address, such as 220.127.116.11. When you have a site, the domain name that you enter in order to open it is for your convenience, however the server where the site files are still has an IP address. Considering that there are many more sites and devices than there are IP addresses, all of the shared web hosting servers have a number of websites under the same IP, whereas using a dedicated server you'll have a dedicated IP too. Even in the first case though, you're able to obtain a dedicated IP for your websites and host them on a shared server. One advantage would be that you may get improved search engine positions since a dedicated IP often means a faster loading site. Furthermore, you need such an IP when you plan to purchase an SSL certificate for your site and protect the info that visitors submit on it.