An IP address is a unique number that distinguishes a site or a server on the world-wide web, so in the event that you have a dedicated IP, it'll be used exclusively by your websites and won't be shared with other people as it happens with shared web hosting accounts. If you have your own web server, you'll have a dedicated IP, but you may need extra ones for different uses. If you have an Internet store, for example, you shall need an SSL certificate for it, to ensure that the payment info that your customers submit shall be encrypted and safe. The same is valid if you have a login form of some kind and you want the usernames and the passwords that visitors enter to be secured. The SSL certificate requires a dedicated IP address, which should be different from the one which you already have on the web server. You may also need a separate IP for an app such as a VoIP server, or if you want a slightly better performance for a particular site, which shall influence its position in search engine results.