All devices on the Web are recognized by a specific number called an IP address, for instance 22.214.171.124. If you have a site, the domain name that you type in order to open it is to save you time, but the server where your site files are still has an IP address. Considering that there are a lot more websites and devices than there are IP addresses, all of the shared web hosting servers have a number of websites under an identical IP, whereas when you use a dedicated server you'll be given a dedicated IP as well. Even in the first case though, you're able to acquire a dedicated IP for your sites and host them on a shared server. An advantage would be that you will get better search engine positions because a dedicated IP generally means a faster loading website. Last but not least, you need such an IP when you intend to acquire an SSL certificate for your website and shield the information that your visitors submit on it.