An IP address is a unique number that distinguishes a site or a web server on the Internet, so if you have a dedicated IP, it shall be used only by your websites and will not be shared with other individuals as it happens with shared web hosting accounts. In case you have your own server, you shall have a dedicated IP, but you may need additional ones for different uses. In case you have an electronic store, for instance, you shall need an SSL certificate for it, to ensure that the payment info your customers submit shall be encrypted and protected. The same is valid if you have a login form of some type and you want the usernames and the passwords that visitors input to be secured. The SSL certificate requires a dedicated IP, which should be different from the one that you already have on the server. You might also need an independent IP for an app such as a VoIP server, or if you want a slightly better overall performance for a given website, which may affect its position in search engine results.