An IP address is a unique number which separates a site or a server on the Internet, so in the event that you have a dedicated IP, it'll be used exclusively by your sites and will not be shared with other people as it happens with shared web hosting accounts. In case you have your own web server, you will have a dedicated IP, but you may need extra ones for a variety of purposes. Provided you have an Internet store, for instance, you'll need an SSL certificate for it, in order to ensure that the payment info your clients submit shall be encrypted and safe. The same is valid if you have a login form of some type and you would like the usernames and the passwords that visitors input to be protected. The SSL certificate requires a dedicated IP address, which ought to be different from the one that you already have on the server. You might also need an individual IP for an app such as a VoIP server, or if you desire a slightly better efficiency for a particular website, which shall affect its position in search engine results.