An IP address is a unique number that separates a site or a web server on the Internet, so if you have a dedicated IP, it'll be employed solely by your websites and will not be shared with other individuals as it happens with shared web hosting accounts. If you have your own hosting server, you shall have a dedicated IP, but you may require additional ones for a variety of purposes. If you have an online store, for example, you shall need an SSL certificate for it, so as to make sure that the payment information that your customers submit shall be encrypted and secure. The same is valid if you have a login form of some type and you would like the usernames and the passwords that visitors type in to be secured. The SSL certificate requires a dedicated IP address, which has to be different from the one that you already have on the machine. You may also need a separate IP for an app such as a VoIP server, or if you would like a slightly better performance for a given website, which may affect its position in search engine results.