An IP address is a unique number which distinguishes an Internet site or a web server on the Internet, so if you have a dedicated IP, it shall be used only by your websites and won't be shared with other people as it happens with shared website hosting accounts. In case you have your own hosting server, you will have a dedicated IP, but you may need extra ones for different purposes. Provided you have an Internet store, for example, you'll need an SSL certificate for it, to make sure that the payment information which your clients submit shall be encrypted and secure. The same is valid in the event that you have a login form of some type and you want the usernames and the passwords which visitors enter 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 might also need a separate IP for an app such as a VoIP server, or if you want a slightly better functionality for a particular website, which shall influence its position in search engine results.