All devices on the Internet are recognized by a specific number called an IP address, for instance 22.214.171.124. In case you have a site, the domain name that you enter to be able to access it is for your convenience, however the server where your site files are still has an IP. Considering that there are a lot more sites and devices than there're IPs, all shared website hosting servers have a number of websites under the same IP, whereas with a dedicated server you'll be given a dedicated IP too. Even in the first case though, you will be able to get a dedicated IP for your websites and host them on a shared server. An advantage would be that you will have far better search engine rankings since a dedicated IP generally means a quicker loading site. Furthermore, you need such an IP in case you want to obtain an SSL certificate for your website and protect the info that your visitors submit on it.