An IP address is a unique number which identifies a site or a server on the world-wide web, so if you have a dedicated IP, it'll be employed exclusively by your websites and won't be shared with other individuals as it happens with shared Internet hosting accounts. If you have your own server, you'll have a dedicated IP, but you may need extra ones for many different uses. Provided you have an online store, for example, you shall need an SSL certificate for it, in order to ensure that the payment information that your clients submit shall be encrypted and protected. The same is valid in case you have a login form of some sort and you want the usernames and the passwords that visitors enter to be protected. The SSL certificate requires a dedicated IP address, which needs to be different from the one that you already have on the machine. You may also need an individual IP for an application such as a VoIP server, or if you desire a slightly better performance for a certain site, which may affect its position in search engine results.