An IP address is a unique number which identifies an Internet site or a server on the Internet, so if you have a dedicated IP, it'll be used only by your sites and will not be shared with others as it happens with shared website hosting accounts. In case you have your own hosting server, you'll have a dedicated IP, but you may require extra ones for many different uses. In case you have an Internet store, for example, you shall need an SSL certificate for it, in order to ensure that the payment info which your customers submit shall be encrypted and safe. The same is valid if you have a login form of some sort and you wish the usernames and the passwords which visitors input to be secured. The SSL certificate requires a dedicated IP, which ought to be different from the one that you already have on the machine. You may also need an independent IP for an application such as a VoIP server, or if you want a slightly better functionality for a certain website, which shall affect its position in search engine results.